— There is somebody at the door, who            it be? Is it the postman?

— No, it            be him, it's just six o'clock. It's too early. [ ]

A. may; can't

B. may; mustn't

C. will; won't

D. must; may not

Humans are social animals. They live in groups all over the world. As these groups of people live apart from other groups, over the years and centuries they develop their own habits and ideas, which form different cultures. One important particular side of every culture is how its people deal with time.
Time is not very important in non-industrial societies. The Nuer people of East Africa, for example, do not even have a word TIME that is in agreement with the abstract thing we call time. The daily lives of the people of such non-industrial societies are likely to be patterned around their physical needs and natural events rather than around a time schedule based on the clock. They cook and eat when they are hungry and sleep when the sun goes down. They plant crops during the growing seasons and harvest them when the crops are ripe. They measure time not by a clock or calendar, but by saying that an event takes place before or after some other events. Frequently such a society measures days in terms of "sleeps" or longer periods in terms of "moons". Some cultures, such as the Eskimos of Greenland measure seasons according to the migration of certain animals.
Some cultures which do not have a written language or keep written records have developed interesting ways of "telling time". For example, when several Australian aborigines want to plan an event for a future time, one of them places a stone on a cliff or in a tree. Each day the angle of the sun changes slightly. In a few days, the rays of the sun strike the stone in a certain way. When this happens, the people see that the agreed-upon time has arrived and the event can take place.
In contrast, exactly correct measurement of time is very important in modern, industrialized societies. This is because industrialized societies require the helpful efforts of many people in order to work. For a factory to work efficiently (well, quickly and without waste), for example, all of the workers must work at the same time. Therefore, they must know what time to start work in the morning and what time they may go home in the afternoon. Passengers must know the exact time that an

By saying "Humans are social animals", the author means ______.

A. they live all over the world
B. they are different from other animals
C. they live in one place, district or country, considered as a whole
D. they are divided into many groups