Calculating the Value of an Economy Is Harder Than Just

Calculating the value of an economy is harder than just adding up the value of every single thing that is produced because:

A) that would lead to overcounting,as the value of intermediate products would be counted twice.
B) that would lead to overcounting,as there is a mark-up in everything that is sold.
C) that would lead to undercounting,as most goods are not sold in retail markets.
D) that would lead to undercounting,as only observable markets can be recorded.