Which of the following is a compound sentence?

1.Turning to look back, we saw how far we had traveled and were satisfied.

2.Laughing wildly, she ran to the door, but she had collected herself before opening it.

3."Onward!” exclaimed my friend, mentally preparing for our hike.Laughing wildly, she ran to the door,
but she had collected herself before opening it

4.In the rear part of the building were two large tables, suitable for carpentry work.