19. Since my professor ____ on the syntactic structures of conditional sentences, I have been able to use them in my daily life.
(A) expounded (B) compounded (C) pounded (D) expended
(A) expounded-to give a detailed explanation to sth. (B) compounded-to make worse (C) pounded-the unit of measuring money/weight (D) expended-to use/spend money, time, or effort