Native English speakers: grammar question

[Martin Moser]

Hi,

someting that came up in an English test of our son and which we are not quite sure of - could I please have your opinion on the tense to be used with the following sentences:

Mr A: "Have you seen the Leinerts?"

a) Mrs A: "No they have left the house in the morning and I think they aren't back yet."

or:

b) Mrs A: "No they left the house in the morning and I think they aren't back yet."

Which one is right, or are maybe both ok?
Thanks for your help!