【单选题】在下列关系代数的操作中,不属于专门的关系运算的是
【单选题】在下列关系代数的操作中,哪一个不属于专门的关系运算
【简答题】Birds are sometimes referred to as glorified reptiles. And even though modern birds are structurally very different from 1 , some show minor evidence of their 2 , like scales on their feet or claws on...
【多选题】Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express important ideas in the passage. Some ...
A.
Although scientists are sure that 600 million years ago glaciers covered the Earth’s continents to the equator, there are many questions about this period of Earth’s history.
B.
Glaciers usually stop spreading because carbon dioxide causes the atmosphere above them to become warmer.
C.
Once the glaciers reached the equator, they immediately began to melt.
D.
Some forms of life must have been able to live through the period of equatorial glaciers.
E.
The equatorial glaciers explain why today’s algae are descended from billion-year-old fossils.
F.
Scientists believe that carbon dioxide is essential to the formation of new glaciers.
【单选题】In addition to a two-party system, there are still several minor parties, such as Libertarian Party, the Green Party and Constitution Party in the United States, __________________.
A.
even though these minor parties have very practical importance
B.
even though these minor parties have some practical importance
C.
even though these minor parties have very little practical importance
D.
even though these minor parties have no practical importance at all
【单选题】当相关系数r=0时,表示两者之间没有关系。( )
【单选题】在下列关系代数的操作中,哪一个不属于专门的关系运算?