An experiment is performed with automobile batteries. Thirty batteries are divided, at random, into three groups of 10. One group will be tested at 15° F, the second group at 70° F, and the third group at 125° F. The useful life of the battery, in hours, will be measured. The same mechanism for testing batteries is used throughout the experiment and the order of testing is randomized. What are the experimental units?
A. The useful life of the batteries
B. The batteries
C. The testing temperatures; 15° F, 70° F and 125° F
D. The mechanism for testing the batteries