The angle is less than 180 degrees, but more than 90 degrees. acute angle obtuse angle triangle right angle

Obtuse 

An acute is less than 90 
Right angle is exactly at 90 

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction