A it is.
1)Y has been believed to cause Z (Y -> Z)
2)A new report, noting that Y and Z are often observed to be preceded by X; ( X -> [Z,Y])
3.)Conclusion :suggests that X, not Y, may be the cause of Z. (X -> Z)
To support new suggestion, Z must be preceded by X.
(A) In cases where X occurs but Y does not, X is usually followed by Z. ( X-> Z); this is true since Y=null
(B) In cases where X occurs, followed by Y, Y is usually followed by Z. (X->Y->Z this violates point 3.
(C) In cases where Y occurs but X does not, Y is usually followed by Z. (Y->Z) this does not strenthen conclusion coz x is missing.
(D) In cases where Y occurs but Z does not, Y is usually preceded by X.(z is missing)
(E) In cases where Z occurs, it is usually preceded by X and Y. (X,Y -> Z) it is reversed and is incorrect.
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