21. A unique feature of capillary electrophoresis is electroosmotic flow (EOF). When a high-voltage is applied across a fused-silica capillary tube containing a buffer solution, electroosmotic flow usually occurs, in which the bulk liquid migrates toward the cathode. What is the polarity of EOF in the presence of chemical modification (cetyltrimethylammonium bromide, CTAB) on the capillary wall?
(A) positive
(B) negative
(C) neural
(D) bipolarity