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.

[EX Q]: 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
[EX A]: D

[EX Q]: Which of these occurs if myosin fails to bind to actin during muscle contraction?  
 (A) The muscle cells will lose ATP. (B) The muscle cells will remain relaxed. (C) The muscle cells will produce lactic acid. (D) The muscle cells will lose calcium ions.
[EX A]: B

[EX Q]: Which of these is the best evidence that Earth's continental landmasses drifted apart over time?  
 (A) the fossil record along the edges of continents (B) the rocky coastlines at the edges of continents (C) the distribution of biomes on all the continents (D) the prevalence of granite on all the continents
[EX A]:
A