This is a periodic problem.

Because its rules are: black, black, white, black, black, white, black, white ... every six is 1 group.

20 12÷6=335.....2

You can think of it this way. You have counted the last color of 335 groups, white, but you counted it again, so it is black.

There are 3 blacks in 1 group and 335×3= 1005 in 335 group.

But the other two are also black.

So there is: 1005+2= 1007.

So choose D.

I wish you progress in your study and hope to adopt it. Thank you! ! O(∩_∩)O~