One of the Most Difficult Problems in Conducting Research on Imagery

One of the most difficult problems in conducting research on imagery is that

A) the individual differences are so large that it's virtually impossible to find any consistent results.
B) it is difficult to examine such an inaccessible mental process.
C) people's introspections about imagery are so vivid that they prevent researchers from manipulating most variables.
D) only a relatively small proportion of people seem to use imagery on a regular basis.