The snow line of the Rocky Mountains of the USA is about

The correct answer is: B. 6 km above sea level.

The snow line is the elevation above sea level at which snow remains on the ground year-round. It is determined by a number of factors, including latitude, altitude, and climate. In the Rocky Mountains of the USA, the snow line is typically around 6 km above sea level.

Option A is incorrect because the snow line is typically much higher than 3 km above sea level in the Rocky Mountains. Option C is incorrect because the snow line is not at sea level. Option D is incorrect because the snow line is not none of the above.