Relevant costing

A company wants to decide whether to make its material in house on whether to subcontract production to an external supplier. In the past it has made for materials in house, but demand in the next year will exceed in-house production capacity of 8000 units. All materials are made on the same machines and required the same machine time per unit: machine time is the limiting production factor.
The following information is available.
Material W X Y Z
Units required 4000 2000 3000 4000

Variable cost $8 Per unit $12 per unit $9 per unit $10 per unit
of inhouse

Directly attributable $5000 $8000 $6000 $7000
fixed cost exp.

Cost of external $9 per unit $18 per unit $12 per unit $12 per unit
purchase

Directly attributable fixed costs are fixed cash expenditures that would be saved if production of the material of the material inhouse has stopped completely

If a decision is made entirely on the basis of short term cost considerations, what materials should the company purchase externally.

A-4000 units of W and 1000 units of Z(according to me)
B-4000 units of W and 4000 units of Z(according to BPP Revision Kit)
C-3000 units of Y and 2000 units of Z
D-1000 units of Y and 4000 units of Z
Sir it has B option, I understood how 4000 of W has come in ans as we made saving of 1000 when we buy externally but I’m not getting the point that how 4000 units of Z we should buy externally as it does not make any saving .