__________ sleep every night is important for teenagers. _________, loss of sleep can make you feel tired, and even cause you to put on weight.

Teachers have to constantly update their knowledge in order to maintain their professional ________.　　　　　　　　　　　　　　　　　　

Bending down，he        the footprints    that hadn’t been blow away in the wind, concluding that they belonged to a wolf.

--- How can I use iPhone 4s?--- Just refer to the _________.

In dealing with public relations, we should make every effort to prevent the _____ in personality.