Answer :
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.'