The Birth Weight of a Newborn Baby Is $6$ Lb $12$ Oz at Its Six-Month Checkup, the Baby's Weight Is

The birth weight of a newborn baby is $6$ lb $12$ oz. At its six-month checkup, the baby's weight is $13$ lb $6$ oz. How much weight has the baby gained since birth?

A) 9 lb $5$ oz
B) $10$ lb $6$ oz
C) $6$ lb $10$ oz
D) $11$ lb $7$ oz
E) $9$ lb 9 oz