If scientists only detect p waves in a certain layer of earths interior what does that tell us?

He want to contact the red cross/red crescent because they are the main non profit organization that deals with disasters around the world 

Humans have survived and thrived in mexico city by using technology to create greater access to land. when the spanish conquered the aztec capital, tenochtitlan, in the 16th century, they built mexico city by draining the lakes and filling in the lake water to create more arable land for agriculture. this technique of land reclamation has continued to be fundamental to human civilization in mexico city. 

Why you put last one a? i think is b.