[問題]一題從ETS的structure sample題

[skull]

One of the most difficult problems in understanding sleep is determining what the functions of sleep is.

我看到答案是is
is改are? 應該吧..

沒看到答案時還以為是determining要改成 to determine
應該Ving跟to V 都可以 對嗎?


Thank you.

[Behemoth]

沒錯
你答對囉