Which sentence is the correctly written compound sentence?
a.
the grand canyon in arizona is 277 miles long.
b.
the grand canyon was carved by the colorado river in the state of arizona, the steep-sided canyon is 277 miles long.
c.
in the state of arizona, the colorado river carved the grand canyon, and it is 277 miles long.
d.
the grand canyon was carved by the colorado river in the state of arizona; the steep-sided canyon is 277 miles long.