Final answer:
The maximum specific thrust for a ramjet occurs at M=0, while the specific thrust at M=20 is lower than the maximum value.
Step-by-step explanation:
In a ramjet, the maximum specific thrust occurs at M=0, which is when the flight Mach number is zero. At this point, the specific thrust is at its maximum value. The specific thrust of a ramjet with subsonic flow in the combustor at M=20 would be significantly lower than the maximum specific thrust at M=0. At M=20, the specific thrust would be reduced due to the increase in flight Mach number.