In a full binary tree if number of internal nodes is I, then number of nodes N are?

In a full binary tree if number of internal nodes is I, then number of nodes N are?
 
(a) N = 2*I
 
(b) N = I + 1
 
(c) N = I – 1
 
(d) N = 2*I + 1
 
This interesting question is from Binary Tree Properties in chapter Binary Trees of Data Structures & Algorithms I
 
The question was asked by my school teacher while I was bunking the class.