Please clarify: to be form

[kiranlegend]

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

In this A is incorrect. Is it incorrect because after to be verb i.e., are a cost is not in plural unlike its subject effects? else are there are any flaws in it?

Alos How is B correct? already cost business more than 100 billion a year.. is more than 100 billion a year an adjectival phrase to modify business?

Please clarify

[Raymott]

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

In this A is incorrect. Is it incorrect because after to be verb i.e., are a cost is not in plural unlike its subject effects? else are there are any flaws in it?

Alos How is B correct? already cost business more than 100 billion a year.. is more than 100 billion a year an adjectival phrase to modify business?

Please clarify

The correct answer should read: " ... which already cost business more than $100 billion a year."
Firstly "which already are a cost" should be "which are already a cost", but I don't think that's the problem.
Perhaps you are right.
In any case, if B is correct, they are stating that
"The effects are not a cost to business." But, "The effects do cost business."
I'm not sure whether that's a difference. Both are grammatical to me.

[kiranlegend]

yeah, probably you are right. I feel A could be wrong due to the prepositional phrase' location. What is your opine?

[Raymott]

yeah, probably you are right. I feel A could be wrong due to the prepositional phrase' location. What is your opine?

Quite possibly, given that I've got no other idea