The following state diagram is given as basis for state transition testing and contains only...

The following state diagram is given as basis for state transition testing and contains only valid transitions:

Explanation of the state diagram: States are depicted as nodes. The initial state is I, the final state is F. A state transition is depicted as a directed arrow with the initiating event as a label, e. g. from I to S1 with event a, and is also written as a triple (I,a,S1). Note: A test case is a sequence of events that initiates the corresponding sequence of state transitions.

The state diagram contains the following 7 state transitions:

(I, a, S1)

(S1, a, S2), (S1, b, S3), (S1, c, F)

(S2, a, S3)

(S3, a, S2), (S3, b, F)

What is the minimum number of test cases to achieve 100% coverage of all valid state transitions in the diagram?