如果 $D=\left|\begin{array}{lll}a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33}\end{array}\right|=2, \quad D_1=\left|\begin{array}{ccc}2 a_{11} & 2 a_{12}-6 a_{13} & 2 a_{13} \\ a_{21} & a_{22}-3 a_{23} & a_{23} \\ a_{31} & a_{32}-3 a_{33} & a_{33}\end{array}\right|$ 则 $D_1=$
A. 8
B. 4
C. 12
D. -21