Agrobacterium Infects Plants and Causes Them to Form Tumors

Agrobacterium infects plants and causes them to form tumors. You are asked to determine how long a plant must be exposed to these bacteria to become infected. Which of the following experiments will provide the best data to address that question?

A) Determine the survival rate of Agrobacterium when exposed to different concentrations of an antibiotic.
B) Measure the number of tumors formed on a plant when exposed to various concentrations of Agrobacterium.
C) Measure the concentration of Agrobacterium in different soil environments where the plants grow.
D) Measure the number of tumors formed on plants, which are exposed to Agrobacterium for different lengths of time.