D
Atoms are all around us. They are the bricks of which everything is made. Many millions of atoms are contained in just one-grain of salt, but despite (尽管) their small size they are very important. The way an everyday object behaves depends on what kinds of atoms are in it and how they act. For example, you know that most solid objects melt if they got hot enough. Why is this? It is the effect of the heat on the object's atoms. All atoms move constantly. When they are hot, they move faster. Usually the atoms in an object hold together and give the object its shape. But if the object grows hot, its atoms move so fast that they break the force that usually holds them together. They move out of their usual places so that the object loses its shape. Then we say that the object is melting.
69. The way an object behaves depends on the _____ .
A. action of atoms in it                      B. number of atoms in it
C. kinds of atoms in it and how they behave    D. size of atoms in it
70. Atoms in an object move _____ .
A. at all times                              B. only when the object is heated
C. whenever they grow hot                    D. unless the object is melted
71. An object holds its shape because its atoms _____ .
A. usually hold together                          B. move fast
C. are hot                                                     D. are not moving
72. An object loses its shape because its atoms _____ .
A. move out                             B. are not in their usual place
C. lose the force                            D. break out