Which storage option should you choose?

You are developing an application to assist the user in conducting electronic surveys.
The survey consists of 25 true-or-false questions.
You need to perform the following tasks:
Initialize each answer to true. Minimize the amount of memory used by each survey.
Which storage option should you choose?

A.
BitVector32 answer = new BitVector32(1);

B.
BitVector32 answer = new BitVector32(-1);

C.
BitArray answer = new BitArray (1);

D.
BitArray answer = new BitArray (-1);

Explanation:
C & D BitVector32 is more efficient than a BitArray when 32 or less binary flags are required.
Primarily because it is a value type.
Note: we are not sure why B is preferred to A.