Lets simplify B :
Here 4 properties are used :
1. a + a' = 1
2. a + ab = a
3. a + a'b = a + b
4 . De Morgans Rule
Given , B = pq' (r's + st' + sr + t + pq'r + r')
= pq'(s + t + st' + pq'r + r')
= pq'(s + t + r' + pq')
= pq's + pq't + pq'r + pq'
= pq'
Hence B' = p' + q