Answer:
A) Nighttime was approaching.
Explanation:
Nighttime was insinuated, but not bluntly or plainly shown. Implicit means "Implied though not plainly expressed." SO if we look at the passage and the multiple-choice questions we see that B, C, and D, were all plainly expressed. They were given a signal, as well as were "explicit" because they didn't hide any information. The only information we have that "nighttime was approaching" would be that the "sky was getting dark."
