When you visit a museum you must ask for _____ before taking photographs inside it. 

Until the start of the Industrial Revolution, mankind was poor, hungry, and constantly at the _______ of disease and natural disasters.

It's hard for him playing against me.I've got nothing to play for，but for him，he needs to win so ________.(2013·湖南)

Mother ______ him not to smoke, but he wouldn’t listen.

 Drinking water , having a healthy snack and taking exercise are all positive ways to ____you when you are working on a tight schedule.