$28 \cdot 27$ Ten trees are lined up in a row, each adjacent pair separated by 10 meters. At the first tree, there is a well, and Xiao Zhang draws water from the well, carries it in a bucket, and waters the trees, with each tree requiring one bucket of water. Then, after watering all ten trees, the minimum distance he has to walk in meters is
(A) 410.
(B) 490.
(C) 500.
(D) 510.
(3rd "Five Sheep Cup" Junior High School Mathematics Competition, 1991)
[Solution] After watering the first and second trees, he needs to walk 20 meters; after watering the third and fourth trees, he needs to walk $20+40$ $\doteq 60$ meters; after watering the fifth and sixth trees, he needs to walk $40+60=100$ meters; after watering the seventh and eighth trees, he needs to walk 60 $+80=140$ meters.
Finally, after watering the ninth and tenth trees, he needs to walk another 90 meters (without returning to the well).
In summary, he needs to walk at least
$$
20+60+100+140+90=410 \text { meters. }
$$

Therefore, the answer is $(A)$.