Part 1

Derek described the whole expression below as a sum of two elements. Which part of the expression could be one of these elements?
1 – a + 3b

A. 1 – a
B. a
C. a + 3
D. b

Part 2

Monica described the whole expression below as a difference of two elements. Which part of the expression could be one of these elements?

1 – a + 3b

A. 1 – a
B. a + 3
C. 1 + 3b
D. 3b