murder in a dome house

A family that lives in a dome house found out that their butler was murdered. When the cops showed up, one of the cops asked the family where they had been. The only people that had been home were the maid, the gardener, the daughter and the son.

The son said he was outside, the daughter said she was in her room doing her homework, the gardnener said he was prunning the bushes and the maid said she was dusting the corners.

Who murdered the butler?