单项选择题

Body Cells People are the most complex of all organisms._________(46). The specialized cells of the body depend upon each other for such things as nourishment and the removal of the waste matter._________(47). Or if the excreting cells did not take away poisonous waste, the cells of the body would be poisoned and die. Under a microscope, a cell looks like a bit of clear jelly with a thin wall round it._________(48 ). In every cell there is a part like a little ball. This is the nucleus, which organizes the work of the cell. Though cells were discovered two and a half centuries ago, it is only in the last hundred years that knowledge of the work of the nucleus has developed._________ (49). All cells reproduce their own kind and grow. In performing their work, cells become tired, worn-out and die. If the tissue of the organ is to maintain its health, new cells are required to replace the old ones. The healthy cells accomplish this by splitting their bodies so that one cell becomes two cells and the two cells four, and so on. These cells are young and small, but, they get nourishment and they grow into adult cells, and so carry on the function of the organ. It is plain, therefore, that each organ of the body must perform its work well if the other organs are to maintain their health. When they are all functioning well, the body is healthy. When any organ fails to do its work, all the organs are liable to become affected; changes take place in their cells and their functions are disordered. _________ (50). Then the body is said to be diseased. A. The cells in the human body work together in various ways. B. And very small openings in the walls of the cell let food in and waste out. C. It is this nucleus which keeps the cell working. D. Such changes give rise to sensations of sickness, pain, loss of strength, bad appetite, fever and so on. E. A man would die, if the millions of cells did not work together in their different ways. F. For example, if the digestive cells of the food canal did not perform their function properly, the muscle cells would not receive nourishment.