Answer:
Following is attached the code that works accordingly as required. It reads in characters from standard input and outputs the number of times it sees an 'a' followed by the letter 'b'. All the description of program is given inside the code as comments.
I hope it will help you!
Explanation:
It is a power-saving mode for a computer.
It is a power-saving mode designed for laptops.
It should be A. DOS because all of the others are operating systems.