What is the output of this program?

#!/bin/sh
test_function1() {
    a=5
    echo "This is the first function"
    test_function2
}
test_function2() {
    echo "This is the second function" 
    test_function3
}
test_function3() {
    echo "This is the third function"
}
test_function1
exit 0

A. This is the first function
This is the second function
This is the third function

B. This is the first function
This is the third function
This is the second function

C. This is the second function
This is the first function
This is the third function

D. This is the third function
This is the first function
This is the second function

Answer: Option A