It’s the third time that Li Ming has been to Xi’an , _______?

all___ for international conferences is of greatimportance．—I see, and we are sure to complete it on time．

To qualify yourself for this kind of new job, I recommend that you _____ some online courses. [ ]

It was not until the beginning of the twentieth century________to explore space.[ ]

They succeeded in killing pests which came from the foreign countries ______ a lot of hard work