The lactone shown is treated with an excess of methylmagnesium bromide, followed by aqueous work-up. Which structure is the product?

Butyrolactone is a cyclic ester. The first Grignard equivalent opens the ring to give a ketone and a primary alkoxide; the ketone is more reactive than the ester and adds a second methyl group, giving a tertiary alkoxide. After work-up the product is 4-methylpentane-1,4-diol, with one primary and one tertiary OH.