Which is a correctly written sentence, not a run-on?

A.
The gate to our backyard had been left open our dogs got out while we were gone.

B.
Our car wouldn't start, so Dad called a tow truck to take it to the mechanic.

C.
Our backyard was all soggy from the rain, the dog's paws were all muddy.

D.
Our cousins are coming for a visit we plan on taking them to the zoo and the museums.