Which of the following sets could not be the lengths of the sides of a triangle? {6, 10, 12} {5, 7, 10} {4, 4, 9} {2, 3, 3}

Remember the following points for such questions:

For a triangle:
1) The sum of lengths of any two sides much be greater than the third side.

2) The difference of lengths of any two sides must be smaller than the third side.

From the above options, only option C {4,4,9} does not obey this rule. As 4 + 4 = 8 and is lesser than the third side.

So {4,4,9} set can not be the lengths of the sides of a triangle