The left most digit of an integer of length 2000 digits is 3. In this integer, any two consecutive digits must be divisible by 17 or 23 . The 2000th digit may be either ' $a$ ' or ' $b$ '. What is the value of $a+b$ ?
(A) 3
(B) 7
(C) 4
(D) 10
(E) 17