A new survey shows that 54 percent of Americans do not take

A new survey shows that 54 percent of Americans do not take prescription medicines as _____ though they are important to their health
A they instruct
B were instructed
C to be instructed
D instructed
选什么,为什么,分析as在句子里的构成,做什么成分,as的用法有哪些啊?