今天看到一题如下，请问要如何计算？
If the computer has 32 data registers (R0 to R31), 2000 words in memory,
and 16 different instructions. What is the minimum size of an add instruction
in bits if a typical instruction use the following format: add R0 R2
(A) 14
(B) 13
(C) 28
(D) 29
答案是给A 请问是如何得出的？
感谢各位！