Answer:
b. to cross over valleys
Explanation:
The Khyber pass, was a mountain pass that's located in Pakistan. Many historians -predicted that This pass was started to be used as trade routes by the Kushan Empire. Prior to that, this pass only used for military's purposes.
After that, The Mauryan empire also start using Khyber pass as their trade routes. It allows their merchants to reach the market in middle east (mostly Pakistan or Afghanistan) at a much faster time compared to using the conventional roads.
