又道题，多谢！！

shala

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
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 cost of
答案为B。没看明白，which在此做什么成分，是修饰abuse的吗？
Thanks !!