A Limitation of the FMRI Technique Used to Study Patterns

A limitation of the fMRI technique used to study patterns of brain activation in the Featured Study by Hamann, Herman, Nolan and Wallen presented in the text was that

A) Computerized Tomography (CT) was only used to create a vivid image of the horizontal slice of the brains of the subjects.
B) Positron Emission Tomography (PET) was used and it does not measure neural activity directly, it only shows areas of increased metabolic activity.
C) Transcranial Magnetic Stimulation (TMS) was used and it can only create three dimensional images of brain structures, not brain activity.
D) functional Magnetic Resonance Imaging (fMRI) was used and it does not measure neural activity directly, it only shows areas of increased metabolic activity.