Why do you think almost a century passed between the development of the first vaccine (for smallpox) and the development of the next vaccine (for cholera)?

Vaccines take a long time to develop and make.

Explanation:

The cure (vaccines) to problems like smallpox take a while to develop as scientists and doctors need to make it correctly and test it correctly to see if it is a cure or not. It takes years before these things can be distributed and/or even verified for "it works!" Due to this, it may of took a century for development, planning, making, testing, getting approved and mass distributing.

the correct answer is:

a population changes in response to a change in the environment.

because the light is a great source of energy for the plants to survive. (i hope this )