Answer the questions on the basis of the information given below:

According to a coding scheme the sentence

Peacock is designated as the national bird of India

is coded as

5688999 35 1135556678 56 458 13666689 1334 79 13366

This coding scheme has the following rules:

1. The scheme is case-insensitive (does not distinguish between upper case and lower case letters).

2. Each letter has a unique code which is a single digit from among 1,2,3, …, 9.

3. The digit 9 codes two letters, and every other digit codes three letters.

4. The code for a word is constructed by arranging the digits corresponding to its letters in a non-decreasing sequence.

Answer these questions on the basis of this information.

Which set of letters CANNOT be coded with the same digit?