Read the following sentences and answer the questi...

Read the following sentences and answer the question below. Taking time off from work is crucial for avoiding stress and depression, and their potential consequences.Graduate students and postdocs, who may equate 'working longer' with 'working better', are particularly prone to working themselves into the ground, says Simon Davy, head of the School of Biological Sciences at the Victoria University of Wellington in New Zealand. Davy, who since his days as a PhD student has vowed not to work on Saturdays, says that he sees students slide easily into working seven-day weeks. Question: What does the author mean by saying "working themselves into the ground "？

A、To exhaust themselves.

B、To lie on the ground.

C、To sleep in the lab.

D、To work from morning till night.