What does riprap symbolize in Gary Snyder’s poem, “Riprap”?

A.the barriers of friendship
B.the needless walls we build around ourselves
C.the journey and the workmanship of the trail of life
D.the things we bother ourselves with in life

I think the answer is C, but I'm really unsure. Please help! I posted before but no one answered!

I saw your post yesterday, and frankly that poem seems mindless to me. Sorry.

I would say that it's either B or D, but I'm not sure.