A Mutant B Cell Line Is Examined by Confocal Microscopy $\theta$

A mutant B cell line is examined by confocal microscopy after incubation with a microbial pathogen recognized by the BCR on these B cells. The B cells have been stained with antibodies to visualize the localization of polymerized actin and microtubules. As a control, wild-type B cells are examined. The results are shown in Figure, with the numbers indicating the proportion of cells examined that show each pattern of staining.
To identify the specific signaling defect in these mutant B cells, a reasonable biochemical assay would be to:

A) Determine if BCR stimulation of mutant B cells produces enhanced binding of the B cell to the microbe
B) Determine whether the mutant B cells have reduced levels of the enzyme Protein kinase C- $\theta$
C) Determine whether the mutant B cells are overexpressing the enzyme Vav
D) Determine whether BCR stimulation of mutant B cells promotes exchange of GDP for GTP on cdc42
E) Determine whether BCR stimulation of mutant B cells produces increased levels of DAG