I don't know why theur friends ( )so happy.括号里填be动词

I don't know why theur friends ( )so happy.括号里填be动词
I don't know why theur friends ( )so happy.
括号里填be动词

I don't know why theur friends ( )so happy.括号里填be动词
are
作为从句用正常语序

are

ARE

are

are were 都可以.