Student or Learner
Martin Hewings' ADVANCED GRAMMAR IN USE 2004 edition CD ROM contains such a dialogue (in unit 98):
--Ken was sacked because he disagreed with his boss.
--Actually, it is because he didn't do any work that he was sacked.
I thought 'was' would be better than 'is' here. But is 'is' also a right choice?
Actually, it was because he didn't do any work that he was sacked.That is good. You could also use is there. (I could attempt an explanation, but it is 3 o'clock in the morning as I type this, and I should be in bed. I'll be headed there soon.)
I hope that helps.