you know that the set can be the sides of a right triangle, if the two shorter sides squared will equal the longer side, or hypotenuse, squared. This is the pythagorean theorem, a^2+b^2=c^2.
For example, A is .3, .4, and .5. Now, as .4 squared plus .3 squared = .5 squared, meaning that these could be the dimensions of a right triangle.
Now, C, the set 2,3,4, when the two legs are squared and added together will yield 13. since 4 is not the square root of 13, this cannot be the dimensions of a right triangle.
See if you can figure out the last one.
