Why Are Coalition Governments So Prevalent in Parliamentary Systems

Why are coalition governments so prevalent in parliamentary systems?

A) Parliamentary systems are nonpartisan,but coalition governments can often act much like parties.
B) The winner-take-all system that is used in parliamentary systems encourages coalition governments.-Consider This: Winner-take-all-system usually creates two party system.
C) Coalition governments discourage third parties.
D) In a multiparty system with proportional representation,single parties usually do not win a majority of the seats.