Use the Following Information to Answer the Question

Use the following information to answer the question.
For the past several decades, amphibian species worldwide have been in decline. A significant proportion of the decline seems to be due to the spread of the chytrid fungus, Batrachochytrium dendrobatidis. Chytrid sporangia reside within the epidermal cells of infected animals, animals that consequently show areas of sloughed skin. The infection cycle typically takes four to five days, at the end of which zoospores are released from sporangia into the environment.
When adult amphibian skin harbors populations of the bacterium Janthinobacterium lividum, chytrid infection seems to be inhibited. Which of the following is the best experimental design to test whether this inhibition is actually due to the presence of J. lividum?

A) Inoculate uninfected amphibians with J. lividum and determine whether the amphibians continue to remain uninfected by chytrids.
B) Inoculate infected amphibians with J. lividum and determine whether the amphibians recover from infection by chytrids.
C) Take infected amphibians and assign them to two populations. Leave one population alone; inoculate the other with J. lividum. Measure the rate at which infection proceeds in both populations.
D) Take infected amphibians and assign them to two populations. Inoculate one population with a high dose of J. lividum; inoculate the other with a low dose of J. lividum. Measure the survival frequency in both populations.