问题 选择题

用显微镜观察血液的涂片,我们看到的血细胞中,数量最多的是(    )

A.血浆

B.红细胞

C.白细胞

D.血小板

答案

答案:B

题目分析:血液分为两部分:血浆和血细胞,血细胞包括红细胞、白细胞、血小板。红细胞呈无细胞核,呈两面凹的圆饼状,成年人每立方毫米血液里红细胞的数量,男子为500万个左右,女子平均为420万个左右;白细胞有多种,有细胞核,比红细胞大,数量少,每立方毫米血液中有5000--10000个;血小板比红细胞和白细胞都小得多,形状不规则,没有细胞核,每立方毫米血液中,血小板的含量为10万--30万个.因此在三种血细胞中红细胞的数量最多

问答题 简答题
单项选择题

Lying in the sun on a rock, the cougar saw Jeb and his son, Tom, before they saw it. Jeb put his bag down quickly and pulled his jacket open with both hands, making himself look big to the cougar. It worked. The cougar hesitated, ready to attack Jeb, but ready to forget the whole thing, too.
Jeb took off his jacket, grasped Tom and held him across his body, making a cross. Now the cougar’s enemy looked even bigger, and it rose up, ready to move away, but unfortunately Tom got scared and struggled free of Jeb.
"Tom, No !" shouted his father.
But Tom broke and ran and that’s the last thing you do with a cougar. The second Tom broke free, Jeb threw himself on the cougar, just as it jumped from the rock. They hit each other in mid air and both fell. The cougar was on Jeb in a flash, forgetting about Tom, which was what Jeb wanted. Cougars are not as big as most people think and a determined man stands a chance, even just with his fists. As the cougar’s claws got into his left shoulder, Jeb swung his fist at its eyes and hit hard. The animal howled(吼叫) and put its head back. Jeb followed up with his other fist. Then out of the corner of his eye, Jeb saw Tom. The boy was running back to help his father.
"Knife, Tom," shouted Jeb.
The boy ran to his father’s bag, while Jeb started shouting as well as hitting, to keep the cougar’s attention away from Tom. Tom got the knife and ran over to Jeb. The cougar was moving its head in and out, trying to find a way through the wall. Jeb was making out of his arms. Tom swung with the knife, into the cougar’s back. It howled horribly and ran off into the mountains.

What do we know about cougars

A.They are afraid of noises.
B.They hesitate before they hit.
C.They are bigger than we think.
D.They like to attack running people.