C come in the picture Because here You need to understand Identical Object Different Box (Star-Bar Problem) Problem In Combinatories
a+b=16 Think like this Suppose we have 16 chocolate and We want to distribute in two children a and b Now we give 5 chocolate a and 7 chocolate to b so total we gave 12 chocolate and Now We have 4 chocolate So where is my 4 chocolate Which is in my pocket
So here C is me because Remanning chocolate C have that’s Why C come in to picture
Note- Please First Understand Identical Object Different Box Problem in Combinatories Chapter in DMGT after that Question is very easy For You.