大全2-12

12.   Y has been believed to cause Z. A new report, noting that Y and Z are often observed to be preceded by X, suggests that X, not Y, may be the cause of Z.

Which of the following further observations would best support the new report’s suggestion?

(A) In cases where X occurs but Y does not, X is usually followed by Z.

(B) In cases where X occurs, followed by Y, Y is usually followed by Z.

(C) In cases where Y occurs but X does not, Y is usually followed by Z.

(D) In cases where Y occurs but Z does not, Y is usually preceded by X.（A）

(E) In cases where Z occurs, it is usually preceded by X and Y.

I'm kind of confused of the answer A.Please give me the hint.Thanks!

old study: Y -->Z

New study: X ---> Y/Z

A) When Y is not present, X--->Z

From the above, we can conclude that X(not Y) ---> Z