Question

4. We must        that the experiments are controlled as rigidly as possible.

   A. assure   B. secure

   C. ensure   D. issue

Answer 1

4. C ensure意为"确保"，相当于make sure; assure意为"向. …保证，使确信，让……放心"；secure意为"使……安全，保护……免受攻击、伤害等"；issue用作动词时，意为"发布，发表，配发"。