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D. 40.0km

Step-by-step explanation:

When rounding to 3 significant figures, we will have to look at the 4th significant figure. If that figure is 1-4, you round the 3rd figure down. If that figure is 5-9, you round the 3rd figure up.

In this case, the 3rd figure is the tenth 9 and the 4th figure is the hundredth 7.

Since 7 is from 5-9, you round the tenth 9 up. This adds one whole to 39, which makes it 40. You also need to keep 3 significant figures so the decimal zeros are significant. Thus, the answer is 40.0km