完形填空
阅读下面短文，掌握其大意，然后在各题所给的四个选项(A、B、C和D)中选出一个最佳选项。
It was a cold January in 1925 in Nome, Alaska. The town was   1 from the rest of the world because of heavy snow.
On the 20th of that month, Dr. Welch 2 a sick boy, Billy, and knew he had diphtheria, a deadly infectious disease mainly affecting children. The children of Nome would be 3 if it spread the town. Dr.Welch needed medicine as soon as possible to stop other kids from getting sick. 4 , the closest supply was over 1,000 miles away, in Anchorage.
How could the medicine get to Nome? Cars and horses couldn't travel on the 5 roads.6 January 26, Billy and three other children had died. Twenty more were 7 .
Nome'stown officials came up with a(n) 8. They would have the medicine sent by train from Anchorage to Nenana. From there, dogsled(狗拉雪橇) drivers—known a"mushers"—would 9it to Nome in a relay(接力)．
The race began on January 27. The first musher, Shannon, picked up the medicine from the train at Nenana and rode all night. 10 he handed the medicine to the next musher, Shannon's face was black from the extreme cold.
On January 31, a musher named Seppala had to 11 a frozen body of water called Norton Sound. It was the most 12 part of the journey. Norton Sound was covered with ice，which could sometimes break up without warning. If that happened, Seppala might fall into the icy water below. He would 13, and so would the sick children of Nome.
A huge snowstorm hit on February 1. The only hope was Balto, Kaasen's lead dog. Balto put his nose to the ground, 14 to find the smell of other dogs that had traveled on the trail. If Balto failed, it would mean disaster for Nome. Ten minutes passed by. Suddenly, Balto began to run. He had found the trail.
At 5：30 am on February 2，Kaasen and his dogs arrived in Nome. Dr.Welch had the medicine. He quickly gave it to the sick children. All of them recovered. Nome had been 15 .