21. Mach number is significant in___________________?

A. supersonics, as with projectiles and jet propulsion
B. full immersion or completely enclosed flow, as with pipes, aircraft wings, nozzles etc.
C. simultaneous motion through two fluids where there is a surface of dis-continuity, gravity force, and wave making effects, as with ship’s hulls
D. all of fhe above
✅ The correct answer is option A.

24. Assertion A.: The determination of pH value of sewerage is important. Reason (R): The efficiency of certain treatment methods depends upon the availability of pH value________________?

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true
✅ The correct answer is option A.