Consider statements about ecology: 1. An ecosystem includes living and non-living components. 2. A community includes only one species. 3. A population includes individuals of the same species. Which are correct? MCQ with Answer and Explanation

Explanation:
The placenta begins to form immediately after implantation of the blastocyst. By the end of the first trimester (around 12 weeks of gestation), the placenta is fully formed and takes over the primary role of hormone production (progesterone and estrogen) from the corpus luteum. It is fully functional in providing oxygen, nutrients, and waste removal for the developing fetus for the remainder of the pregnancy. It is not fully formed at conception, nor does it wait until the second or third trimester.

Explanation:
In C4 plants like maize and sugarcane, CO₂ is initially fixed in mesophyll cells by PEP carboxylase, producing a four-carbon compound oxaloacetic acid (OAA), which is then converted to malate or aspartate and transported to bundle sheath cells. Here, CO₂ is released and enters the Calvin cycle. This adaptation minimizes photorespiration.

Explanation:
A food web is a complex network of interconnected food chains showing multiple feeding relationships in an ecosystem. Decomposers (bacteria and fungi) break down dead organic matter, returning essential nutrients to the soil for producers. Due to energy loss at each transfer (10% law), the total biomass generally decreases at higher trophic levels, forming an upright pyramid. All statements are correct.