【2020最新整理国开(电大)人文英语1-3896网上形考机考题库】They must try to _________ the boundaries of knowledge.

【2020最新整理国开(电大)人文英语1-3896网上形考机考题库】They must try to _________ the boundaries of knowledge.
选择一项：
A. extend
B. increase
C. enrich