做DNA粗提取和鉴定实验时，实验材料用鸡血而不用猪血的原因是[ ]
A．鸡血的价格比猪血的价格低
B．猪的红细胞没有细胞核，不易提取到DNA
C．鸡血不凝固，猪血会凝固
D．用鸡血提取DNA比用猪血提取操作简便
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阅读理解。
This is a boy. His name is Tom. He has a nice cat. It is an English cat. His friends are Lucy and Mike.
They have cats, too. Their names are Mimi and Maomao. They are Chinese cats. The English cat is five.
The Chinese cats are three. They like their cats.
Today they can't find the English cat and the Chinese cats. They ask an old man, "Where are our cats?" "I don't know." He says, "Are they in the tree?" Tom and Mike say. "Listen, listen!" Lucy says.
"Mi
根据短文内容，判断下列句子正（T）误（F）。
( )1. Tom has a good English cat.
( )2. Tom has four friends.
( )3. The English cat is three and the Chinese cats are five.
( )4. Tom and his friends have six cats.
( )5. Their cats are in a tree.
国土90%的土地为沙漠，是一个典型的沙漠之国的国家是[ ]
A．埃及
B．美国
C．法国
D．澳大利亚
每人每天装订图书420册，照这样计算，4人5天能装订图书多少册？列综合算式是（ ）。
压力蒸气灭菌时物品的装载错误的是（）
A.尽量将同类物品放在一起灭菌
B.物品的上下左右均应间隔一定距离
C.难灭菌的大包放上层，易灭菌的小包放下层
D.金属物品放下层，织物包放上层
E.玻璃瓶等开口向上或侧放
为了对危险源进行()管理，防止重大事故发生，提出了重大危险源的概念。
A.责任
B.分级
C.预防
D.分类