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设(X1，X2，…，Xn)为取自正态总体X～N(μ，σ2)的样本，则μ2＋σ2的矩法估计量为

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设X，Y是两个随机变量，且P｛X≤l，Y≤1｝＝4／9，P｛X≤1｝＝P｛Y≤1｝＝5／9，则P｛min(X，Y)≤1｝＝()．

A. 4／9
B. 20／81
C. 2／3
D. 1／3

A．coursesB．placesC．courtsD．playground

A. courses
B. places
C. courts
D. playground

商业信用是工商企业之间提供的信用，它以借贷活动为基础。()

A. 正确
B. 错误

Los Angeles has planted 2,000 rubber trees down the middle of one of its main streets. These trees do not【21】rubber. They are,【22】, made of rubber. Mr. Joe Dynamo, a spokesman for Los Angeles【23】council, explained the reasons. He said:
"These【24】are representative of our virtual society. We have polystyrene grass on our golf【25】. We have non-milk powder【26】our coffee. We【27】copies in plastic of old wooden furniture. We have fiberglass tombstones.【28】shouldn't we have trees made of rubber？"
"At 50 miles an hour,【29】motorist will see any difference. And our preservation【30】will be lower. You give the trees an annual rinse (清洗) with detergent, and【31】them off twice a year. We【32】a lot of money because transplanting, pruning, weeding and leaf-collection are all【33】. And we have a 60-year【34】guarantee on each plant. This is rationalization at【35】best. "
(41)

A. invent
B. do
C. produce
D. create