If there are N firms in the economy, each assigned with a serial number from 1 to N, then as per the product method, GDP can be written as ______.

A

$$GVA_1 - GVA_2 ...... - GVA_N$$

B

$$NVA_1 + NVA_2 ...... + NVA_N$$

C

$$GVA_1 + GVA_2 ...... + GVA_N$$

D

$$GVA_1 + NVA_2 + GVA_3 + NVA_4 ....$$