Consider the following statements:
In the case of convergent nozzle for compressible flow,
1. No shock wave can occur at, any pressure ratio.
2. No expansion wave can occur below a certain pressure ratio.
3. Expansion wave can occur below a certain pressure ratio.
4. Shock wave can occur above a certain pressure ratio.
Which of the following statements given above are correct?
a. 1 and 2 b. 3 and 4
c. 1 and 3 d. 2 and 4