6、Through a series of experiments an American scientist has gained an understanding of the social structure of the most complex of ant societies. The ants examined are the only creatures other than man to have given up hunting and collecting for a completely agricultural way of life. In their underground nests they plant gardens on soils made from finely cut leaves. This is a complex operation requiring considerable division of labor. The workers of this type of ant can be divided into four groups according to size. Each of the groups performs a particular set of jobs.   

The making and care of the gardens and the nursing of the young ants are done by the smallest workers. Slightly larger workers are responsible for cutting leaves to make them suitable for use in the gardens and for cleaning the nest. A third group of still larger ants do the construction work and collect fresh leaves from outside the nest. The largest are soldier ants, responsible for defending the nest.

To find out how good the various size - groups are at different tasks, the scientist measured the amount of work done by the ants against the amount of energy they used. He examined first the gathering and carrying of leaves. He selected one of the size groups, and then measured how efficiently these ants could find leaves and run back to the nest. Then he repeated the experiment for each of the other size groups. In this way he could see whether any group could do the job more efficiently than the group normally undertaking it. 

The intermediate-sized (中等的) ants that normally perform this task proved to be the most efficient for their energy costs, but when the scientist examined the whole set of jobs performed by each group of ants, it appeared that some sizes of worker ants were not suited to the particular jobs they performed .

1．According to the passage, the ants______.

A．grow something in their nests             B．go hunting for a living

C．cut leaves to make a fire                  D．do each of the jobs all together

2．It is observed that slightly larger ants perform more of the______.

      A．construction tasks                      B．defensive work

      C．household tasks                         D．dangerous work

3．The underlined word "good" means ______.

      A．co-operating       B．efficient            C．hardworking      D．responsible

4．The experiments made by the scientist were based on ______.

      A．special methods                         B．scientific theories

       C．personal interests                    D．systematic observations