题目

用括号内所给动词的适当形式填空，必要时加助动词。      Salt is very common in our everyday life, but have you ever   1   (hear) of some stories about salt? Talking about salt, perhaps nobody can   2   (tell) exactly when people first began   3   (use) it. But it is known to us all that salt  4  (use) in many different ways all through history. People who   5   (live) over 3,000 years ago ate salted fish and salted meat. In ancient Egypt(古埃及), salt was used to keep dead bodies.      6  (Steal) salt was regarded as a serious crime(罪行) a不透明的口袋里装有白、黄、蓝三种颜色的乒乓球（除颜色外其余都相同），其中白球有2个，黄球有1个，现从中任意摸出一个是白球的概率为. （1）试求袋中蓝球的个数. （2）第一次任意摸一个球（不放回），第二次再摸一个球，请用画树状图法，求两次摸到都是白球的概率.