A class has a ratio of 3:6:7 of children with red hair, blonde hair and black hair respectively. If the class has 9 kids with red hair, how many kids are there with black hair?	Choose the correct choice from the following answers:
A) 7
B) 6
C) 9
D) 21
E) 63
Since there is a 3:7 ratio between red haired children and black-haired children, if there are 9 children with red hair, then there must be 3 times the amount of black-haired children. Therefore, are 21 children with black hair.   Answer: 21)
<result>D</result>
A class has a ratio of 3:6:7 of children with red hair, blonde hair and black hair respectively. If the class has 9 kids with red hair, how many kids are there with black hair?