Respuesta :
It's B, the writer is halfway to the top as at the same time "a sharp wind gusts up from the deep glacier-cut valley" it is also stated that "I glanced up at the summit" and then they say that it would take a while, implying that the writer is about to continue climbing up to the top of the trail.
B. hike to the top of the trail is what the writer is about to do. The writer glances up ( he takes a quick look upwards) at the summit ( the top of the mounatin) after evaluating some information. The lenghth of The Chilkoot ; the fact that the pass ( a road through the mountain) is some miles in elevation.
The underlined words indicate the direction the writer will take: to the top of the trail.