Each of four persons fires one shot at a target. Let C_k denote the event that the target is hit by person k, k = 1, 2, 3, 4. If C₁, C₂, C₃, C₄ are independent and if P(C₁) = P(C₂) = 0.7, P(C₃) = 0.9, and P(C₄) = 0.4, compute the probability that 

(a) All of them hit the target

(b) Exactly one hits the target

(c) No one hits the target

(d) At least one hits the target.