In English, the subject of a command, order, or suggestion — you, the person being directed — is usually left out of the sentence and is said to be the understood subject:
[You] Step lively there or I'll leave you behind! Before assembling the swingset, [you] read these instructions carefully.
I don’t know if this is what you’re looking for, but I hope I cleared things up for you.
I believe the correct answer is the second choice: <span>We walked through the Red Oak Nature Center today; it was surprising how many bicyclists were there. </span>
