Acontrolled experiment is one that
a. proceeds slowly enough that a scientist can make careful records of the result.
b. tests experimental and control groups in parallel.
c. is repeated many times to make sure the results are accurate.
d. keeps all variables constant.

The correct answer is B. Tests experimental and control groups in parallel

Explanation:

In science, a controlled experiment is a type of experiment in which scientist intervene or manipulate variables. This often implies scientists use a control group in which the variable is constant and an experimental group that has a different treatment or changes in the variable studied. This allows scientists to evaluate the effect of the variable by comparing the results of both groups and thus confirm or discard a hypothesis. Therefore, a controlled experiment can be defined as on that "tests experimental and control groups in parallel".

b. the carbon cycle ensures living things have a source of hydrogen