Dr. Blehert took 117 healthy bats into his laboratory. He exposed 83 of them to the fungus suspected of causing white-nose syndrome (WNS) while the rest of the bats were not exposed to the fungus but were otherwise kept under the same environmental conditions. All of the bats were monitored for symptoms of WNS. The intentionally manipulated independent variable in Dr. Blehert’s experiment was
Choose one:
A. the number of healthy bats at the beginning of the experiment.
B. development of WNS symptoms in the healthy bats.
C. the number of bats affected by the fungus.
D. exposure to the fungus.