UNIX-Operating-System

The contents of the text file t1.txt containing four lines are as follows:

a1 b1
a2 b2
a3 b2
a4 b1 

The contents of the text file t2.txt containing five lines are as follows:

a1 c1
a2 c2
a3 c3
a4 c3
a5 c4 

Consider the following Bourne shell script:

awk - F''' {Print S1, S2} ' t1.txt |
while read a b ; do
            awk -v aV = Sa - v bV = Sb - F''
            'aV = = S1 (print aV, bV, S2 )'t2.txt
done 

Which one of the following strings will NOT be present in the output generated when the above script in run? (Note that the given strings may be substrings of a printed line.)