[20 Points]
Add the expressions 4- 2/3b+ 1/4a and 1/2a + 1/6b -7 . What is the simplified sum?
A: -1/6a + 5/12b - 3
B: 3/4a + 1/2b - 3
C: 3/4a + 5/6b - 3
D: 5/12a - 1/6b - 3