A program encrypts a message using a Caesar-style alphabetic shift cipher.

The letters of the alphabet are shifted a set number of places. A positive shift moves the letters to the right (towards Z/z). A negative shift moves them to the left (towards A/a).

Spaces, punctuation, symbols, and numbers must not be encrypted.

When the end of the alphabet is reached with a shape shift, wrapping must occur so the alphabet acts as a loop. For example, a shift of +3 encodes 'z' to 'c', and a shift of -2 encodes 'a' to 'y'. Case is preserved throughout encryption.

The program must produce the encrypted ciphertext shown in the test table below:

Table 1: Caesar Cipher Test Cases

Below is an incomplete Python implementation of this program. It contains five sections where you must choose between alternative lines of code.

def encrypt_message(plain_text, shift):
    cipher_text = ""
    for char in plain_text:
        # --- CHOICE 1 ---
        # Option A1: if char.isalpha():
        # Option A2: if char.isalnum():

            # --- CHOICE 2 ---
            # Option B1: start = ord('a') if char.islower() else ord('A')
            # Option B2: start = ord('z') if char.islower() else ord('Z')

            # --- CHOICE 3 ---
            # Option C1: position = ord(char) - start
            # Option C2: position = ord(char)

            # --- CHOICE 4 ---
            # Option D1: new_position = (position + shift) % 26
            # Option D2: new_position = (position + shift) % 52

            # --- CHOICE 5 ---
            # Option E1: cipher_text += chr(new_position + start)
            # Option E2: cipher_text += chr(new_position)
        else:
            cipher_text += char
    return cipher_text