When does the embryo form?
A. At the end of the first trimester
B. In the second trimester.
C. A few days after fertilization
D. At the end of the third trimester.

C. A few days after fertilization.

Explanation:

During the first two weeks, the embryo moves through the uterine tube to the uterus where it will implant. The differentiation that establishes the organ systems takes place between 3 and 8 weeks in the first 8 weeks following fertilization.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."