og10 Executives and federal officials say that the use of crack and cocaine is g

wjjason2012

1. Executives and federal officials say that the use of crack and cocaine is growing rapidly among workers, significantly compounding the effects of drug and alcohol abuse, which already are a cost to business of more than $100 billion a year.
A. significantly compounding the effects of drug and alcohol abuse, which already are a cost to business of
B. significantly compounding the effects of drug and alcohol abuse, which already cost business
C. significantly compounding the effects of drug and alcohol abuse, already with business costs of
D. significant in compounding the effects of drug and alcohol abuse, and already costing business
E. significant in compounding the effects of drug and alcohol abuse, and already costs business

请问c中的错误是由于with么，具体怎么分析c呢，然后b选项的which修饰的是the use of crack and cocaine，对不对，这样感觉修饰的好远

soulwangh

1/
Yes, you are right about the antecedent of "which", because cost is a plural verb.

N1 prep N2 + which+Verb In this pattern, which can applies to either N1 or N2, depending on whether Verb is singular or plural.  
2/
C is wrong because Comma+With pattern modifies the whole preceding clause.
But the intended meaning is to modify the effects.
Moreover,  cost of $ is not idiomatic.