The next exam will be more difficult than can be imagined.这个句子中,“than can be imagined”

问题描述:

The next exam will be more difficult than can be imagined.这个句子中,“than can be imagined”
“than can be imagined”叫什么成分?是否应该改为than what can be imagined?than引导宾语从句?

The next exam will be more difficult than (an exam that)can be imagined.