Answer:
Here's what I get.
Explanation:
Consider the molecule shown below.
It has two chiral centres, labelled R and S, and an internal mirror plane that makes the right and left halves of the molecule mirror images of each other.
If we pass a beam of plane-polarized light through the sample, the R portion of the molecule will rotate the light in a certain direction.
However, the S portion will rotate the light by the same amount in the opposite direction.
The two effects cancel each other, so the molecule has no net effect on plane-polarized light.
Even though the molecule has two chiral centres, it is achiral because it has an internal plane of symmetry.
