What relationship could scientists track in order to see whether or not blue crabs were the main reason that large zebra mussels have declined?

A. the relationship between the size of zebra mussels and the size of blue crabs over one year


B. the relationship between the average numbers of large zebra mussels and blue crabs over time


C. the relationship between the average numbers of blue crabs and phytoplankton over time


D. the relationship between the size of blue crabs and the size of zooplankton over one year