I need help with these two questions

Describe how gravity and convection work together to move the plates in the plate tectonics theory

An externality can be created by a. people working at a production job and enjoying their work b. a production activity but not a consumption activity c. a consumption activity but not a production activity d. a production activity or a consumption activity externalities a. are always positive, and an example is flu vaccinations b. are always negative, and an example is the pollution created by trucks c. can be negative or positive. logging has a negative externality, and education has a positive externality d. can be negative or positive. a negative externality is having to go to school when you would rather go to the beach, and a positive externality is spending time with your friends

According to the aztec history, how did the aztecs know where to build their new city of tenochtitlan? why was this a good location?