单项选择题 We all know that normal human daily cycle of activity is of some 7-8 hours' sleep alternating with some 16-17 hous' wakefulness and that, broadly speaking, the sleep normally（） the hours of darkness.

单项选择题 His red hair and short（）made him easy to recognize.

单项选择题 Professor White, my respected tutor, frequently reminds me to（）myself of every chance to improve my English.