由于装卸搬运的作业内容复杂多变，，因此装卸搬运环节成为提高物流系统效率的关键所在。
参考答案：对
He tries _______ English. And he finishes______ homework every day.
A．to learn, to do
B．learning, doing
C．to learn, doing
D．learning, to do
简述约翰穆勒的价值理论。
