Help: a LR Question

It is illogical to infer a second and different effect from a cause which is known only by one particular effect. This is incorrect because the inferred effect must necessarily be produced by some different characteristic of the cause than is the observed effect, which already serves entirely to describe the cause.

Which one of the following arguments makes the same logical error as the one described by the author in the passage?

(A) An anonymous donor gave a thousand dollars to our historical society. I would guess that that individual also volunteers at the children's hospital.

(B) The radioactive material caused a genetic mutation, which, in turn, caused the birth defect. Therefore, the radioactive material caused the birth defect.

The answer is (A).

But I think (B) is a better choice. Any thoughts?