rel T1 with n (2000) tuples and x (80) blocks
rel T2 with m (400) tuples and y (20) blocks
If Nested-loop join algorithm is used to perform the join:
R join S access cost = { X+N*Y } blocks
= 80+2000*20
=40080 blocks
S join R access cost = { Y+M*X } blocks
=20+400*80
=32020 blocks
but If Block nested-loop join is used to perform the join:
R join S access cost = { X+X*Y } blocks
= 80+80*20
=1680 blocks
S join R access cost = { Y+Y*X } blocks
=20+20*80
=1620 blocks
reduction in number of block accesses required for reading the data = 32020-1620
=30400
so ans should be B