Omar observes that many buildings in his city were built using limestone. He has read that acid rain can damage limestone. He also knows that limestone reacts with acids and that chemical reactions are affected by temperature. With this in mind, he conducts an investigation to see how the amount of damage to the limestone is affected by the amount of acid on the stone.

Which statement describes the correct plan for his procedure?

A. The temperature and the amount of damage to the limestone are controls. The amount of acid and the type of limestone are variables.

B. The temperature and the type of limestone are controls. The amount of acid and the amount of damage to the limestone are variables.

C. The amount of acid and the type of limestone are controls. The temperature and the amount of damage to the limestone are variables.

D. The amount of acid and the amount of damage to the limestone are controls. The type of limestone and temperature are variables.

The correct plan for Omar's procedure is to keep the temperature and type of limestone constant (controls), and vary the amount of acid applied to measure the resulting damage to the limestone (variables).

Omar's investigation into how acid rain affects limestone requires a clear experimental design. Controls are aspects of the experiment that are kept constant to ensure that the results are due to the manipulated variable. In this case, both the temperature and the type of limestone should be controls because we want to study how just the amount of acid influences damage to the limestone. The variable that Omar changes is the amount of acid applied to the limestone, and the amount of damage to the limestone is what he measures. Therefore, the correct statement for planning his procedure is: 'The temperature and the type of limestone are controls. The amount of acid and the amount of damage to the limestone are variables.'