背景障植物的富集倍数通常（） A.大于300倍 B.30－300倍 C.30倍以上

选择与下列各题中画线部分意思相同或相近的选项。

1. Marica called him three times yesterday,but nobody answered.

A. visited

B. told

C. phoned

2. My cousin gave up the chance of working abroad and returned to his hometown.

A. opportunity

B. challenge

C. experience

3. He finds watching English movies frustrating because the people speak too quickly.

A. boring

B. difficult

C. disappointing

4. Don't blame your friends. Actually they don't know the truth,either.

A. In all

B. In fact

C. In general

5. The child nearly used up all his pocket money.

A. ran out of

B. kept out of

C. got out of

从地形条件分析，世界人口稠密的地区主要分布在：（   ）

A．高原

B．丘陵

C．山区

D．平原

巨幼细胞性贫血患者可选择的治疗药物是（）

A.叶酸和（或）维生素B₁₂

B.口服琥珀酸亚铁

C.丙酸睾酮

D.免疫抑制剂

E.造血细胞生长因子

工人站在3m高的平台上，用绳子将质量为20kg的筐砖从地面吊上来，如图所示，第一次用30s，第二次用60s，则工人所做的功和功率（　　）

A．第一次做功多，功率大

B．第二次做功多，功率大

C．两次做功一样多，功率一样大

D．两次做功一样多，第一次功率大

钢丝绳通常进行哪两个项目的试验？