APMOPS 2011 Question 10

If numbers are arranged in three rows $A,B,C$ in the following manner, which row will contain the number 1000?

$A$: 1, 6, 7, 12, 13, 18, 19, ...

$B$: 2, 5, 8, 11, 14, 17, 20, ...

$C$: 3, 4, 9, 10, 15, 16, 21, ...