Four machines, a, b,c and d, are connected on a computer network. it is feared that a computer virus may have infected the network. your security team makes the following computer virus statement:

1. if d is infected, then so is c

2. if c is infected , then so is a

3. if d is clean, then b is clean but c is infected.

4. if a is infected, then either b is infected or c is clean.

assume that these statements are all true, what can you conlude?