You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

[EX Q]: Which best describes sexual reproduction in all animals?
(A) Egg and sperm join together. (B) Pollen and seed join together. (C) Offspring have traits of only one parent. (D) Offspring are identical to one parent.
[EX A]: A

[EX Q]: Which action creates competition between environmental concerns and economic gains?
(A) protecting endangered wildlife (B) reintroducing wildlife into their natural habitats (C) using forest land to build homes (D) using city buses for transportation
[EX A]: C

[EX Q]: Animals can affect their habitats through activities of finding food and raising their young. Sometimes these activities have a positive effect on the habitat of other organisms. Which is the best example of an activity that has a positive effect?
(A) elephants stripping bark from trees (B) viruses infecting the cells of certain birds (C) beavers constructing dams to make ponds for lodges (D) semi-aquatic rodents feeding on the roots of aquatic plants
[EX A]:
C