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.

During a biochemical reaction, pyruvate (C3H4O3) is converted into carbon dioxide (CO2) and water (H2O). In what cellular organelle does this biochemical reaction occur?  
 (A) nucleus (B) cytoplasm (C) mitochondria (D) endoplasmic reticulum
C

Shale is a sedimentary rock that can be metamorphosed into slate by  
 (A) cementation. (B) chemical weathering. (C) sedimentation. (D) increased pressure.
D

Over time, iron becomes rust when it combines with water and oxygen. Which type of change must occur for rust to form?  
 (A) chemical (B) mechanical (C) nuclear (D) physical

A