Practice questions without fighting the page.

The verb beware is always followed by the preposition of when it means be cautious about / be on guard against something or someone dangerous or harmful.

Structure: beware of + noun/pronoun/gerund

<b>Correct answer: B. Stratosphere</b> <ul> <li>The ozone layer is located in the <b>stratosphere</b>, roughly 10–50 km above Earth's surface, peaking around 20–30 km.</li> <li>Key concept: O3 absorbs harmful UV radiation. Formation: O2 + hv → 2O; O + O2 → O3.</li> <li>Exam tip: Remember “Ozone = Stratosphere, not Troposphere.”</li> <li>Why others are incorrect: Troposphere is where weather occurs; Mesosphere is higher and thinner; Magnetosphere is not a atmospheric layer.</li> </ul>

<b>Correct option: B</b> - Radio waves <ul> <li><b>Reason</b>: Radar uses radio waves to detect aircraft by sending a radio pulse and measuring its reflection (echo) from objects.</li> <li><b>Key concept</b>: Electromagnetic waves in the radio frequency range travel long distances and can reflect off metal objects like planes.</li> <li><b>Simple formula</b>: Range ~ (speed of light × time delay)/2; time delay between transmitted and received pulse gives distance.</li> <li><b>Exam tip</b>: If a question mentions detection of distant or fast-moving objects, think radio waves (radar).</li> <li><b>Why others are incorrect</b>: <ul> <li>Sound waves: attenuate in air and require detectors near the object; not used for long-range detection.</li> <li>Ultrasonic waves: high frequency, short range.</li> <li>Infrared: limited range and weather sensitivity; not suitable for radar surveillance.</li> </ul> </li> </ul>

Use 'had I spoken'