分析流行病学中，研究者可以控制选择偏倚是在（）。 A.研究设计时 B.结果分析时 C

若抛物线y²=8x的焦点是F，准线是l，则经过点F、M（3，3）且与l相切的圆共有 [ ]

A．0个

B．1个

C．2个

D．4个

古城小学有一个长60m，宽40m的游泳池。选用哪个比例尺画出的平面图最大？选用哪个比例尺画出的平面图最小？[ ]

A.1：2000

B.1：1500

C.1：1000

根据短文内容, 从下框的A～F选项中选出能概括每一段主题的最佳选项, 选项中有一项为多余项。

A. The history of the city

B. The city attracted all kinds of immigrants and developed its multicultures.

C. The brief introduction to New Orleans

D. The origin of Jazz

E. The population of the city

F. African people arrived and they brought their own traditions including music.

     1.________

     New Orleans, the city of Louisiana, is one of the most colorful and interesting places in North

America. It is wellknown for its multicultural cuisine(菜肴) and its colonial architecture and is

considered by most experts to be the birthplace of jazz, the one music style that the USA can lay

claim to inventing.

     2. ________

     The city was founded in 1718, at a time when France owned that part of North America. Part of its

diversity results from the fact that it was subjected to many different influences. France sold Louisiana

to Spain in 1763, got it back 40 years later, and almost immediately sold the territory to the USA.

     3. ________

     As the city prospered(繁荣), it became a magnet(磁铁)for all kinds of immigrants from Europe and

other parts of the USA, and was also the first choice destination of breeding black slaves. The rich

cultural mix led to a relaxed attitude to life and an appreciation of good food, wine, music and dancing.

There were frequent festivals.

     4. ________

     By the end of the 1700s, people of African descent(祖先) made up more than half the city's

population. Many more arrived via the Caribbean and brought with them West Indian cultural

traditions. By the mid18th century, black people  in New Orleans were gathering socially on Sundays

at a special market outside the city, where they were able to remember and practice their traditional

African dance and drumming music. The roots of jazz can be traced to these events.

     5. ________

     Native American musicians then combined their music with the African traditions and bands playing

a mixture of styles began to perform in street festivals. Europeans came to listen and contribute and the

roots of jazz were established. Louis Armstrong, the most famous jazz musician of all time, was

influenced by these rich musical mixtures when he heard them in the early years of the 20th century.

流水线寄存器文件

在有一定催化剂及混合气组成时，氨合成反应出现（）时的温度，称为最适宜温度。

A.最大浓度

B.最大速度

C.最大压力

D.最大速度和最大压力