The main disadvantage of a longitudinal design is

A) too much expense.
B) too much time involvement.
C) both too much time and expense.
D) None of these.

Longitudinal designs require data to be collected over a prolonged period, which can be both time-consuming and expensive. This makes longitudinal studies difficult to conduct compared to other types of research designs. Therefore, the best choice is C which reflects both time and expense as the disadvantage of a longitudinal design.