求解释一道SAT语法题：
问题描述：
求解释一道SAT语法题：
Many alcoholics attempt to conceal their problem from their fellow workers,( but invariablyfailing to keep their secret.)
A.but invariablyfailing to keep their secret.
B.but they invariably fail to keep their secret.
D.who invariably fail to keep their secret.
答
平行结构.sd do ,but do D.指代错误