根据直径是圆中最长的弦，知该圆的直径是10cm；最短弦即是过点P且垂直于过点P的直径的弦；根据垂径定理即可求得CP的长，再进一步根据勾股定理，可以求得OP的长．
—Look! There are so many people in the restaurant.
—Yes. The food there ________ be delicious.
—____________ we leave the library before 5 o'clock?
—No, you needn't. You can stay here until 5:30.
The biggest challenge weall face is to prevent further environmental disasters. We ________ take action before it is too late.
Listen up, guys!Everyone ________ wear a mask. Go and buy one if you don’t have one.
—Must I return this book before Friday, Sir?
—No, you ________,dear. You can keep it for two weeks.
You’d better take an umbrella when you go out, because it ________rain.
The man _______ be a doctor in this hospital, but I’m not sure.
—Whose notebook is this?
—It ________ be Alan’s. It looks like his.
Mary ________ go to London next year but she is not sure.
