听力原文: Whales--or their ancestors--were once land animals. Scientists believe that they roamed the earth about 150 million years ago.
The land-dwelling ancestors of this modem king of the sea were hunters with legs and the jaws and teeth of killers. Their favorite hunting grounds were probably shallow waters near the mouths of rivers or off a level stretch of coast, for fish, both finny and shell, were then more plentiful in the water.
Ages passed, and it became easier for these creatures to swim than to walk; millions of years later they were able to do without legs altogether. Now their hind legs had shrunk so that mere traces of them can be found under the skin when a whale is dissected.
(33)
A.The largest sea creatures ever known.
B.Fish living in shallow waters.
C.Plant-eating land animals.
D.Meat-eating land animals.
第1题
若关系R和S的关系代数操作的结果如下,这是执行了()。
A.R∞S
B.R∞S
C.R∞S
D.R∞S
第2题
假设有如下的关系R和S:
若关系R和S的关系代数操作的结果如下,这是执行了 ______。
A.R×S C<E
B.R×S C>E
C.R×S R.B=S.B
D.R×S
第3题
若结果如下,这是对关系R和S执行了
A.R∞S
B.R∞S
C.R∞S
D.R∞S C<E C>E R.B=S.B
第4题
下列问题的关系代数操作基于如下的关系R和S:
若关系R和S的关系代数操作的结果如下,这是执行了()。 [*]
A.R∞S
B.R∞S
C.R∞S
D.R∞S
第5题
A.R∞S
B.R∞S
C.R∞S
D.R∞S
第6题
下面两题的关系代数操作基于如下的关系R和S。
若关系R和S的关系代数的操作结果如下,这是执行了
A.
B.
C.
D.
第8题
若关系R和S的关系代数操作的结果如下,这是执行了 ______。
A.R×S C<E
B.R×S C>E
C.R×S R.B=S.B
D.R×S
第10题
若关系R和S的关系代数操作的结果如下,这是执行了()。
A.RS
C<E
B.RS
C>E
C.RS
R.B=S.B
D.RS
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