Relation of parent and subsidiary

Sir there was a question like this:

Pigeon Co is a UK registered company and has a wholly owned subsidiary, Duck Co in the USA, which specialises in disposal of nuclear waste. Duck Co is being sued by residents in the USA, for personal injuries following exposure to nuclear waste from its disposal plants. As Pigeon Co is the holding company and is highly profitable, the residents feel that it is Pigeon Co that is liable for the personal injuries suffered.

Is Pigeon Co liable to pay damages for personal injuries on behalf of Duck Co, and for what reason?

A. Yes, because it is head of the group and English law treats groups of companies as single economic units
B. Yes, because Duck Co is acting as an agent of Pigeon Co
C. No, because Duck Co is a separate legal entity with all the rights and liabilities which would normally be attached to a separate legal entity
D. No, because Pigeon Co has no direct presence in the USA

Why is the answer C?

The textbook says

In Adams v Cape Industries plc 1990, three reasons were put forward for identifying the companies as
one, and lifting the veil of incorporation. They are:
-The subsidiary is acting as agent for the holding company.
-The group is to be treated as a single economic entity because of statutory provision
-The corporate structure is being used as a facade (or sham) to conceal the truth.

So the group is to be treated as a single economic entity, why is it not A?