Which phrase from the passage best helps the reader to understand the meaning of the phrase “keenly observant”?

The man flung a look back along the way he had come. The Yukon lay a mile wide and hidden under three feet of ice. On top of this ice were as many feet of snow North and south, as far as his eye could see, it was unbroken white, save for a dark hair–line that curved and twisted from around the spruce–covered island to the south, and that curved and twisted away into the north, where it disappeared behind another spruce–covered island. . . . 2 But all this made no impression on the man. It was not because he was long used to it. He was a new–comer in the land and this was his first winter. The trouble with him was that he was without imagination Fifty degrees below zero was to him just precisely fifty degrees below zero. That there should be anything more to it than that was a thought that never entered his head. . . . 3 At the man’s heels trotted a dog, a big native husky The animal was depressed by the tremendous cold. It knew that it was no time for travelling. Its instinct told it a truer tale than was told to the man by the man’s judgment. . . . The dog had learned fire, and it wanted fire. . . . 4 Empty as the man’s mind was of thoughts, he was keenly observant, and he noticed the changes in the creek, the curves and bends and timber–jams, and always he sharply noted where he placed his feet. . . . 5 And then it happened. At a place where there were no signs, where the soft, unbroken snow seemed to advertise solidity beneath, the man broke through. It was not deep. He wetted himself half–way to the knees before he floundered out to the firm crust. 6 He was angry, and cursed his luck aloud for he would have to build a fire and dry out his foot–gear. This was imperative at that low temperature—he knew that much; and he turned aside to the bank, which he climbed. On top, tangled in the underbrush about the trunks of several small spruce trees, was a high–water deposit of dry firewood. . . . He threw down several large pieces on top of the snow The flame he got by touching a match to a small shred of birch–bark that he took from his pocket. . . . 7 He worked slowly and carefully, keenly aware of his danger. Gradually, as the flame grew stronger, he increased the size of the twigs with which he fed it. He squatted in the snow, pulling the twigs out from their entanglement in the brush and feeding directly to the flame. He knew there must be no failure. When it is seventy–five below zero, a man must not fail in his first attempt to build a fire—that is, if his feet are wet. . . . 8 There was the fire, snapping and crackling and promising life with every dancing flame. He started to untie his moccasins. They were coated with ice; the thick German socks were like sheaths of iron half–way to the knees; and the moccasin strings were like rods of steel all twisted and knotted For a moment he tugged with his numbed fingers, then, realizing the folly of it, he drew his sheath–knife. 9 But before he could cut the strings, it happened. It was his own fault or, rather, his mistake. He should not have built the fire under the spruce tree.......... Now the tree under which he had done this carried a weight of snow on its boughs It grew like an avalanche, and it descended without warning upon the man and the fire, and the fire was blotted out!