Read the statements given below and choose the correct option : Assertion (A)…

Dissolved-oxygen availability governs breathing (ventilation) rate. Air contains roughly 20% oxygen by volume, while even well-aerated water holds only a small fraction of that concentration dissolved in it. Because their surrounding medium supplies far less oxygen per unit volume, water-breathing (aquatic) animals must move a much larger volume of that medium across their respiratory surfaces (gills) per unit time than air-breathing (land) animals need to move air across their lungs. A lower oxygen supply in the medium therefore drives a higher, not lower, ventilation rate.

Checking Assertion (A): it claims the breathing rate of aquatic animals is slower than that of land animals. This runs opposite to the governing principle above — aquatic animals actually breathe faster to compensate for the oxygen-poor medium, so (A) is false. Checking Reason (R): as worded, it asserts that the presence of large amounts of dissolved oxygen is what affects the breathing rate of aquatic animals — that is, it takes for granted that water actually carries a large amount of dissolved oxygen. That underlying premise is what must be checked, independent of whether such a mechanism would be plausible if it held: water in fact carries far less dissolved oxygen than air, not a large amount, so the premise is false and (R) is false on its own terms. Since both the assertion and the reason are factually incorrect, the option recording that both (A) and (R) are false is the one that matches the biology.

• Both (A) and (R) true, with (R) explaining (A): this pairing needs the assertion's breathing-rate claim to be accurate. Since a low-dissolved-oxygen medium drives faster, not slower, ventilation in aquatic animals, that requirement fails.

• Both (A) and (R) true, with (R) not explaining (A): this still treats the assertion's breathing-rate claim as accurate, differing only on whether the reason explains it. The same reversed-direction error undoes it.

• (A) true, (R) false: this correctly senses a problem with the reason's oxygen-amount claim but still keeps the assertion's breathing-rate direction as true — the same low-oxygen-drives-faster-ventilation fact that undermines the reason also undermines the assertion itself.

Cross-check: the CTET July 2024 official Paper II answer key (Set I) and the NCERT Class 10 Science chapter on Life Processes both treat the low dissolved-oxygen content of water — and the correspondingly faster gill-ventilation rate of aquatic animals — as the standard fact.