一块金属的质量为9×10³kg，它的体积是3 m³ ，则这金属的密度是_________；若将它截去一半，则其质量为_________，体积为_________，密度为_________．

Did you ever have someone’s name on the tip of your tongue and yet you were unable to recall it (1) this happens again, do no (2) to recall it. Do something (3) for a couple of minutes, (4) the name may come into your head. The name is there since you have met (5) person and his name. It (6) has to be dug out. The initial effort to recall (7) the mind for operation, but it is the subconscious (8) that go to work to dig up a (9) memory. Forcing yourself to recall (10) never helps because it doesn’t (11) your memory；it only tightens it. Students find the preparatory method help (12) examinations. They read over the questions (13) trying to answer any of them. (14) they answer first the ones (15) which they are most confident. Meanwhile, deeper mental activities in the subconscious mind are taking (16) ; work is being done on the (17) difficult questions. By the time the easier questions are answered, answers (18) the more difficult ones will usually begin to (19) into consciousness. It is often (20) a question of waiting for recall to come to the memory.

20（）

A.nearly

B.likely

C.just

D.even