The Great Wall of China is 374 of age hope this help
Sound wave is a pressure caused by the vibration of something in a medium that transfers anergy, like air.
Environment related opportunities for service include lake cleanup. The correct option among all the options that are given in the question is the first option or option "a". There are several other types of services also related to environment. I hope that this answer has come to your help.
Watershed is probabley the answer you are looking for