以网络计划起始节点为开始节点的工作，当未规定其最早开始时间时，其最早可能开始时间为零。

A. 对
B. 错

The teacher should not demonstrate to let the students understand the purpose and intention in the meaningful practice.

A. 对
B. 错

Teachers should gradually reduce the control of students because their role is mainly to guide, supervise, monitor and evaluate in the meaningful practice.

A. 对
B. 错

The teacher should create a relaxed and lively atmosphere conducive to the development of students' creative thinking in the meaningful practice.

A. 对
B. 错

The teacher should pay more attention to coherence and expressiveness of language rather than authentic in the meaningful practice.

A. 对
B. 错

The teacher should not control the discipline in class to ensure all students to practice in order in the meaningful practice.

A. 对
B. 错

下列说法正确的是 。

A. 熔、沸点高低排序:SrO >BaO > MgO
B. 熔、沸点高低排序:K2O > CaO > PH3 > NH3
C. 熔、沸点高低排序:CaO > K2O > PH3 > NH3
D. 熔、沸点高低排序:BaO < SrO < MgO

下列各物质化学键中只存在离子键的是 。

A. MgCl2
B. CaCO3
C. SiO2
D. C2H4

合同履行的原则包括全面和____。

当事人应当按照约定____履行自己的义务。