Me ajudem, por favor? Está muito difícil

5 - Marque a opção que tenha o mesmo significado da conjunção destacada: a) We have so many problems in the world, BUT one of the most serious is the global warming. ( ) then ( ) so ( ) because ( ) however b) Our city is very dirty BECAUSE a lot of people throw garbage on the street. ( ) but ( ) as a result of ( ) instead of ( ) and Conjunção destacadas:But, because