Read the following passage carefully and answer the questions that follow. In a language laboratory, students were given an interesting puzzle involving the word ”ELECTROCARDIOGRAPH.” The teacher explained that such exercises not only test logical skills but also sharpen attention to detail. According to the challenge, the word had to undergo a series of transformations.

First, the class was asked to take the first half of the letters, reverse their order and make the arrangement of letters look quite different from the original. Next, the students were told to identify the last but one letter of the original word and place it at the very beginning, a step that changed the opening appearance of the sequence completely. Finally, as a finishing touch, they had to add the letter ’S’ at the end. Following these steps carefully would lead them to the correct transformed word, and only those who adhered to each condition in the exact order could solve the puzzle successfully. (150 words)

Which letter is prefixed to the word after the first half is reversed?