Comparative or Superlative ?

They say Chinese is one of ____________ (difficult) languages to learn.

Speak __________________(loud). I can't hear you.

That was _____________(bad) thing you could have told her.

He is one of _________________(rich) men on earth.

Rock is _________________ (popular) type of music among teenagers.

My English teacher is ________________ (friendly) my geography teacher.

Which country is __________ (big) : Canada or the USA?

You have to learn to be ________________ (careful).