Which of the following is a reason why complex political systems developed in river
valleys?

2cool and cold temperature unless its an ever green tree 

D. tropical is the right answer, tropical climate zone is the one who has the least change in temperature between winter and summer months.

1. why do astronomers think that there may be other advanced civilizations?
because  it was discovered that there are other planets like earth out there, able to  accommodate life forms.
2. what technologies are astronomers using to look for life on other planets? why?  
they  have used radio telescopes, but now they use laser telescopes   to track laser communications.