Question

The dotted lines show how piano wire

is used to slice this cube. What will be
the shape of the cross section?

Answer 1

C. rectangle (not a square)

Explanation:

So image that the wire just sliced that piece of the cube right off:

It would give you a triangular prism,

HOWEVER, this is asking for the cross section:

The part of this triangular prism that was actually connected to the cube

and in this case, it would show that the bottom of the cut off part is rectangle.

You can also think of this as: What shape would be left over on the cube?

Cutting this piece off would give you a shape left over that is a rectangle.