instruction:
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.
question:
If an atom of hydrogen gives up one electron, what is its overall charge?  
 (A) -1 (B) +1 (C) -2 (D) +2
answer:
B


question:
Two layers of sedimentary rock are exposed on the side of a hill. Only one of the layers has fossils. The lack of fossils in one layer of sedimentary rock is most likely due to  
 (A) changing environments. (B) volcanic activity. (C) changing rates of erosion. (D) evaporation of sea water.
answer:
A


question:
Around areas of increased fertilizer runoff, all of the heterotrophs within a lake food chain may show an increase in numbers. After the initial increase, the populations begin to decline. Which event most likely caused the populations to decline?  
 (A) More predators were introduced into the lake. (B) Water was depleted of nutrients such as nitrogen. (C) Breeding season for the organisms came to an end. (D) Decomposing algae lowered the amount of dissolved oxygen.
answer:

D