Which of the following is an example of a run-on sentence?
OA.
Stephanie Rhyndon walks dogs after school; doing so is her new part-time job.
OB
The city plans to build new tennis courts at the park many athletes will be happy.
OC.
Because his electricity bill perplexed him, Mr. Arnold scratched the top if his head.
OD. My neighbor's cat strolls into our backyard every morning, but I'm not sure why.