ＧＲＥ全真考試題四

　A volunteer uses a truck to pick up donations of unsold food and clothing from　stores and to deliver them to locations where they can be distributed. He　drives only along a certain network of roads.　

　　In the network there are two-way roads connecting each of the following pairs　of points: 1 with 2, 1 with 3, 1 with 5, 2 with 6, 3 with 7, 5 with 6, and 6 with 7.　There are also one-way roads going from 2 to 4, from 3 to 2, and from 4 to 3. There　are no other roads in the network, and the roads in the network do not intersect.　

　　To make a trip involving pick-ups and deliveries, the volunteer always takes a　route that for the whole trip passes through the fewest of the points 1 through　7, counting a point twice if the volunteer passes through it twice.　

　　The volunteer s home is at point 3. Donations can be picked up at a　supermarket at point 1, a clothing store at point 5, and a bakery at point 4.　Deliveries can be made as needed to a tutoring center at point 2, a distribution　center at point 6, and a shelter at point 7.　If the volunteer starts at the supermarket and next is to go to the shelter,　the first intermediate point his route passes through must be

　　2

　　3

　　5

　　6

　　7

　　答案：(B)

　　If starting from home, the volunteer next is to make pickups for the shelter　at the supermarket and the bakery (in either order), the first two intermediate　points on his route, beginning with the first, must be

　　1 and 2

　　1 and 3

　　2 and 1

　　2 and 4

　　4 and 2

　　答案：(A)

　　If starting from the clothing store, the volunteer next is to pick up bread at either　the supermarket or the bakery (whichever stop makes his route go through the　fewest of the points) and then is to go to the shelter, the first two points he　reaches after the clothing store, beginning with the first,

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