Yes it's fine except for the output should also have

D[0]
D1
DA

as I need exact matching i.e. only D in input will not delete D1 and DA from output.

And in the addition code, if I dont want the new outfile to be sorted.And it's a simple appending the strings whatever they be without any string matching of file B with file A.

So in the addition requirement if A contains

B
C
F[0]

and B contains

D[0]
E

output should be 

D[0]
E
B
C
F[0]