下列事件：（1）阴天会下雨；（2）随机掷一枚均匀的硬币，正面朝上；（3）12名同学中，有两人的出生月份相同；（4）2008年奥运会在北京举行．其中不确定事件有（　　）

A．1个

B．2个

C．3个

D．4个

How should you signal the crane operator to stop in an emergency?（）

A.Extend arm with the palm down and hold this position rigidly

B.Place clasped hands in front of your body

C.Extend arm and move hand rapidly right and left with the palm down

D.Place both fists in front of the body with the thumbs pointing outward