配制（）密度液和进行浮沉时，操作人员穿戴好防护用品，使用橡胶手套、围裙和防护眼镜。
A.磁铁矿粉
B.苯
C.氯化锌
D.氧化铁
参考答案：C
Though this has been written _____ the agreement, it is not protected _____ law.
A．by; in
B．in; with
C．in; by
D．by; with
核酸分子中核苷酸之间的连接键是______。
A．2’,3’磷酸二酯键B．2’,5’磷酸二酯键C．3’,5’磷酸二酯键D．肽键E．δ-1,4糖苷键
