General Science MCQ Practice

• Correct answer: C – Hydrogen sulphide. Brass (Cu with Zn) tarnishes in air containing H2S, forming copper sulfide (CuS) on the surface, causing dark discoloration.
• Key concept: sulfide corrosion (reaction of copper with H2S in air).
• Simple idea: exposure to sulfur-containing gases leads to a surface chemical change, not oxidation by O2 alone.
• Brief note on options: Al₂S₃ or AlP are not typical air pollutants causing brass tarnish; “Hydrogenated wafers” is irrelevant.
• Exam tip: Remember common indoor pollutants like H2S cause dark tarnish on copper alloys.

• Correct answer: D — Plants
• Reason: Primary producers convert solar energy into chemical energy via photosynthesis (concept: energy flow in ecosystems).
• Key concept: Photosynthesis equation (simplified): CO2 + H2O + light energy → C6H12O6 + O2.
• Why others are incorrect: A. Bacteria can be producers in some ecosystems (chemosynthesis) but not the common primary producers; B & C are irrelevant human categories.
• Exam tip: Remember “producer” = makes organic matter; “plants” are the classic primary producers in most ecosystems.

• Reason: Nitrogen is a fundamental element in all amino acids, the building blocks of proteins. Proteins contain N in amino groups (-NH2).
• Key concept: Nitrogen balance and the essential presence of N in proteins, not in carbohydrates, fats, or vitamins in the same abundant form.
• Note: For proteins, the general formula is not fixed, but amino acids always include nitrogen.
• Exam tip: Remember “N” is characteristic of proteins among biomolecules.
• Incorrect options: Carbohydrates and fats supply energy; vitamins are nitrogen-free or have limited N in some, but not a defining constituent of all.

• Reason: Persistence of vision is the brain’s tendency to retain an image briefly after the object has moved, causing a sequence of still pictures to appear as continuous motion — the basis of cinema.
• Key concept: This is a perceptual illusion that enables motion pictures from rapid succession of frames.
• Formula/logic (brief): If frame rate > ~16–24 frames per second, motion appears smooth to the eye.
• Exam tip: If a question asks about moving images created from still frames, think of cinema/film.
• Why others are incorrect: A camera captures light; a binocular is for depth perception; a periscope uses mirrors—none rely on persistence of vision for motion perception.

• Malaria is caused by Plasmodium parasites transmitted by the bite of Anopheles mosquitoes, which are active mainly at night.
• Key concept: vector-borne disease; specific vector species determine transmission.
• Why others are incorrect: Aedes transmits dengue, Zika; Culex transmits filariasis and some encephalitis; not responsible for malaria.
• Exam tip: Remember “Anopheles for malaria” as a quick association.

• Reason: Cuscuta (dodder) is a parasitic plant that lacks independent photosynthesis and obtains nutrients from a host via specialized structures called haustoria.
• Key concept: Parasitism in plants (heterotrophy via a host).
• Why others are incorrect:
  • Saprophyte: feeds on decaying organic matter, not live host tissues.
  • Epiphyte: grows on another plant for support but does not extract nutrients from it.
  • Xerophyte: adapted to dry environments, not about parasitism.

Exam tip: Look for structures like haustoria to identify parasitic plants.

• Why correct: Adrenaline (epinephrine) is released in emergencies to rapidly prepare the body for fight-or-flight, boosting heart rate, blood flow to muscles, and glucose release.
• Key concept: It is a catecholamine from the adrenal medulla that triggers immediate stress responses.
• Formula/numeric logic: Quick rise in cAMP and second-messenger signaling enhances glucose mobilization (no complex math needed).
• Exam tip: If a question mentions rapid, immediate protective action to danger, think adrenaline.
• Why others are incorrect: Insulin lowers blood glucose over time; thyroxine regulates metabolism more slowly; vasopressin controls water balance and vasoconstriction but not labeled as “emergency hormone.”