Read the following poem, then answer the question that follows:

Reginald wanted perfect grades,
and it almost worked out that way.
Math class was his Achilles' heel,
In this course he'd never earn an A.

This poem contains a mythological allusion to the Greek hero Achilles. Using the poem as a guide, what does an Achilles' heel refer to?

An easy subject
A favorite thing
A single weakness
A surprising event