contest R2 Q5

p, q and r are three non-negative integers such that p + q + r=10. The maximum value of pq + qr+ pr + pqr is  : a)  ≥40 and < 50 b) ≥ 50 and < 60 c) ≥ 60 and < 70 d) ≥ 60 and < 80