Aftersomanydayswithoutrain,thegroundquickly________thelittlerainthatfelllastnight.

Theaddressbook________theemailaddressesofyourfriendsorgroupsofcolleagues.

已知离散型随机变量X的概率分布为

X10203040

P a

(1)求常数a;(2)求X的数学期望EX.

Thosewho’dliketovisittheexhibition________(sign)yournameshere.

EachyearintheUnitedStates,manyblackteenagers_______ofschool,eitherbecausetheycannotkeepuportheyhavetoworktosupporttheirfamily.