如图，PA为⊙O的切线，A为切点，⊙O的割线PBC过点O与⊙O分别交于B、C，PA=8cm，PB=4cm，求⊙O的半径．

选择下列词汇完成下面的对话，并在答题卡上将相应的字母编号涂黑。（共5小题，每小题1分）

Lisa: Lily, it's Saturday today. Why not go and ____  ourselves this evening?
Lily: Good idea! What would you like to do? See a movie or watch a play?
Lisa: A movie. It's said a new movie ____ The Grey is on these days. Have you seen it?
Lily: No. But I heard it was the____  movie of the year.
Lisa: Really? Then how about the film Shadows of Love? It's a love story. It should be ____ .
Lily: OK. What time does it ____ , do you know?
Lisa: Wait a minute. I'll check it... Yes. It says 7:30 this evening.
Lily: Good. See you then.

下列是人体健康的有关问题，错误的是（ ）