题目内容：
根据下面资料，回答题 Jack Billabong is a stockman. One Friday afternoon he was riding along the track towards the
Henderson farm. He was looking for a prize bull which had escaped from the Borrogee paddock (小围场). He reached the hills and saw at once that he could not go further.
There had been a fire in the forest which had gone out. But the air was still full of smoke andfallen trees had blocked the track. Jack was just going to turn back when he saw something movingin the smoke. He waited. It was a girl on a horse, and she was riding towards him. "There' s abadly burnt man on the farm," the girl shouted. "He saved my life. Please help me to save him. "
The girl was Cathy Henderson. She had been on horseback for two hours. She had to jump overfallen trees that were still burning. Her pony had fallen with her twice. She was thirsty and almost
dead from want of sleep. But she rode back to the farm with Jack immediately.
Joe Brook was unconscious when they reached him. They lifted him onto Jack' s horse. The
ride back to Borrogee was terrible. Cathy was so tired that Jack had to tie her to her pony. The ponywas tired too, but its courage was astonishing. It followed Jack right to Borrogee Hospital. Nobodysaw them arrive because it was night.
"I've never seen a horse like that pony," Jack said. Cathy praised Jack Billabong, but sherefused to say anything about the pony, "Joe will write about her in his story," she said.
But she did say one thing: "If Flame hadn' t returned to the farm that afternoon, Joe and Iwould have died. "
Jack Billabong did not turn back because________. A．he hadn' t found the bull yet
B．he smelt a heavy smoke in the forest
C．he knew there was a burnt man
D．he saw something moving towards him
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