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.

The teacher heats a pot of water. Once the water reaches 100°C, which point has the water reached?  
 (A) boiling (B) melting (C) freezing (D) evaporating
A

A student notices a paw print while walking in a park. The paw print could be considered a fossil if the print was in ___.  
 (A) rock (B) topsoil (C) thick mud (D) loose sand
A

Which of these provides most of the oxygen in Earth's atmosphere?   
 (A) coral reefs (B) rainforest plants (C) ocean phytoplankton (D) continental grasslands

C