Question

Which sentence does not contain any errors in comma usage?

A.
Before the sun set, I brought in the chairs, that I had left in the yard.

B.
Yes I already sent a thank-you card to Grandma.

C.
Walking along the path I hummed a song to myself.

D.
In the back of the closet, you'll find a box of old clothes.

I'm torn between a and d. Thanks so much for your time and help.