图论中的图不仅反映了研究对象之间的关系，而且是真实图形的写照，因而对图中点与点的相对位置、点与点连线的长短曲直等都要严格注意。
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网络最短路问题和网络最小树问题实质是一个问题。
A. 对
B. 错
When we were kids, we went to Beijing for our holidays, but I _______ there for years.
A. have not been
B. am
C. have been
D. had been
We lived in Hunan from 1995 to 2001, but we _______ to Hainan now.
A. have moving
B. have moved
C. moved
D. was going to move
They did not come to our party yesterday, even though we ________them special invitation.
A. sent
B. send
C. had sent
D. were sent
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