关系代数运算是以集合运算为基础的运算，它的基本操作是 【4】 、并、差。

According to a UN report, one third of the world population have no _________ to clean drinking water and health care.

A．means

B．approaches

C．ways

D．access

根据句意、汉语或首字母提示，补全单词，完成句子。

1. Loud and noisy music often makes me         (不舒服)while I’m eating.

2. S___ _ studies have shown that music can affect people's mood.

3. That young lady in a p        dress looks very pretty.

4. The stone is very h        and it can hardly be torn up easily.

5. How many customers does this restaurants        a day?

感染中毒型钩体病的临床特点有（）。

A.浅表淋巴结肿大伴压痛

B.肌肉疼痛以腓肠肌特别明显

C.以上都是

D.睑结膜充血，但不伴畏光及分泌物

E.发热、头痛、酷似流行性感冒

粘膜大量浆液渗出属（）

A．蜂窝织炎

B．脓肿

C．卡他性炎

D．纤维素性炎

E．积脓

电网频率过高、过低对汽轮机有什么影响？