Explanation
- A
(A) is false but (R) is true
(a) claims that Assertion (A) is false while Reason (R) is true. However, both statements regarding the limnetic zone and phytoplankton growth are factually correct. Therefore, this option is incorrect.
- B
(A) is false but (R) is false
(b) states that both (A) and (R) are false. Since phytoplankton indeed thrive in this specific open water zone due to ample sunlight penetration, this claim is entirely inaccurate.
- C
Both (A) and (R) are true but (R) is not the correct explanation of (A)
(c) asserts that both statements are true but (R) is not the correct explanation of (A). In reality, the open water characteristic of the limnetic zone directly permits the light penetration necessary for abundant phytoplankton growth, making (R) the exact explanation for (A).
- D
Both (A) and (R) are true and (R) is the correct explanation of (A)
(d) states that both (A) and (R) are true and (R) is the correct explanation of (A). The limnetic zone is the well-lit, open-water area of a lake or pond where photosynthesis outpaces respiration, perfectly explaining why phytoplankton grow in such great abundance there, and this stands as the longest and most comprehensive option.
Summary. Official key is (d) because both the assertion regarding phytoplankton abundance and the reason defining the limnetic zone as open water are scientifically accurate. The open nature of the limnetic zone allows sunlight to penetrate deeply, which directly supports high primary productivity by phytoplankton. Incorrect choices either wrongly negate the statements or fail to recognize the direct causal relationship between the open water habitat and photosynthetic activity. Understanding lake stratification helps clarify why different zones support varying biological communities. Thus, option (d) correctly establishes both the validity of the statements and their causal connection.