Why Is It Generally Considered Tentative to Describe What Infants

Why is it generally considered tentative to describe what infants are thinking in research about causation?

A) Researchers can only base their inferences on what the infants say to them.
B) Researchers can make inferences but do not really know what an infant is thinking.
C) Researchers cannot make inferences because habituation tasks are inconclusive.
D) Researchers often use older children to tell them what it is infants are thinking.