instruction:
You are given a science question (easy-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.
question:
The North America Nebula is named for its similarity in shape to the North American continent. It was discovered within the Milky Way by astronomer William Herschel in 1790. What was Mr. Herschel most likely observing?  
 (A) the explosion following the collapse of a star (B) a dense cluster of stars of varying magnitudes (C) meteorites as Earth passed through the tail of a comet (D) a cloud of hydrogen gas and dust where stars are born
answer:
D


question:
Which two processes occur during the life spans of all animals?  
 (A) hibernation and migration (B) growth and development (C) changing color and shedding fur (D) building nests and communication
answer:
B


question:
Which substance contributes most to the greenhouse effect?  
 (A) wood smoke (B) nitrous oxides (C) carbon dioxide (D) nuclear radiation
answer:

C