To a worker, ________ from the point of view of society is necessary labor is from his own point of view voluntary play.
A
本题要求填入一个关系代词引导主语从句，并在主语从句中作主语，所以选what。
Thechallengeisnotoneofexpansion._______,therapidgrowthinenrollmentoverthelast40yearshascometoanend.
The challenge is not one of expansion. _______, the rapid growth in enrollment over the last 40 years has come to an end.
The traffic was held up for more than thirty minutes, ________ caused me to arrive late.