单选题 Mary spent at least two hours ______ her lessons every day before the exam.

单选题 With ______ about 4,000 tigers remaining in the world today, time is quickly running out for this beautiful animal.

单选题 By the time I left the school I ______ that course for ten years.

单选题 When ______ the English level of Japanese and Chinese secondary pupils, the professor gave no comment.

单选题 An online degree course allows you to ______.

单选题 If he ______ hard last term, he ______.

单选题 We will mark each box with a different color ______ we can easily tell one box from another.

单选题 Not until evening came ______ that he hadn’t done half of his work.