B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
A student examines a new watch design. The watch has a small solar cell on its face. The main function of the solar cell is to

### Choices:
A. reduce the cost of the watch.
B. provide an energy source for the watch.
C. make the watch more attractive.
D. stop the watch from speeding up.

### Answer:
