Choose the correct alternative answer for the following question.
If two sides of a triangle are 5 cm and 1.5 cm, the length of its third side cannot be __________
According to the triangle inequality, "the sum of the lengths of any two sides of a triangle must be greater than or equal to the length of the third side".
Therefore, the length of the third side cannot be greater than 3.4 cm.
Hence, the correct answer is 3.4 cm.