system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
#!/usr/bin/python3

'''

a modified version of cvim_server.py orginially
created by 1995eaton
https://github.com/1995eaton/chromium-vim/blob/master/cvim_server.py

USAGE: ./cvim_server.py If you want to use native
Vim to edit text boxes you must be running this
script. To begin editing, first map the
editWithVim (e.g. "imap <C-o> editWithVim")
mapping.

-- additional notes from budRich

VB4C_VIM_COMMAND, which defaults to the
environment variable with the same name. Or "gvim
-f" if not set.

This version will instead execute a command if the
first word posted in a message to the server
matches EXEC_WORD and exectute the rest of the
message as a command.

below is an example from my cvimrc 
(VB4C_EXEC_WORD = brwscon)
that will execute 'notify-send' with a URL as the only
argument. (S : current page, s : from hint)
...

let vimport = 8064

externalCommand(link) -> {{

  const COMMAND   = 'notify-send'
  const EXEC_WORD = 'brwscon'

  const FIELD_SEPARATOR  = "\x1c"
  const URL = (link ? link.href || link.value : document.URL)

  let args = [
    COMMAND,
    URL,
  ]

  cmd = EXEC_WORD
  for (let arg of args) {
    cmd += arg.replace(/^.+$/,FIELD_SEPARATOR + arg)
  }

  PORT('editWithVim', { text: cmd })

}}

map S :call externalCommand<CR>
map s createScriptHint(externalCommand)
...

for the commands in the example to work, this script
must be running and executed something like this:

(backslash escapes new line)

VB4C_PORT=8064         \
VB4C_EXEC_WORD=brwscon \
./vb4c_server.py

'''

import os
import sys
import shlex
from json import loads
import subprocess
from tempfile import mkstemp
from http.server import HTTPServer, BaseHTTPRequestHandler


VB4C_PORT = int(os.getenv("VB4C_PORT", 8001))
VB4C_VIM_COMMAND = os.getenv("VB4C_VIM_COMMAND", 'gvim -f')
VB4C_EXEC_WORD = os.getenv("VB4C_EXEC_WORD", 'vb4cconnect')


def edit_file(content):
    fd, fn = mkstemp(suffix='.txt', prefix='cvim-', text=True)
    os.write(fd, content.encode('utf8'))
    os.close(fd)
    subprocess.Popen(shlex.split(VB4C_VIM_COMMAND) + [fn]).wait()
    text = None
    with open(fn, 'r') as f:
        text = f.read()
    os.unlink(fn)
    return text


class Vb4cServer(BaseHTTPRequestHandler):

    def do_POST(self):
        length = int(self.headers['Content-Length'])
        content_str = self.rfile.read(length).decode('utf8')
        content = loads(content_str)
        self.send_response(200)
        self.send_header('Content-Type', 'text/plain')
        self.end_headers()

        brwschk = content['data'].split(u'\x1c')
        edit = ''

        # Block XMLHttpRequests originating from
        # non-Chrome extensions
        if not self.headers.get('Origin', '').startswith('chrome-extension'):
            pass

        elif brwschk[0] == VB4C_EXEC_WORD:
            subprocess.Popen(brwschk[1:])

        else:
            edit = edit_file(content['data'])

        self.wfile.write(edit.encode('utf8'))


def init_server(server_class=HTTPServer, handler_class=BaseHTTPRequestHandler):
    server_address = ('127.0.0.1', VB4C_PORT)
    httpd = server_class(server_address, Vb4cServer)
    httpd.serve_forever()


try:
    init_server()
except KeyboardInterrupt:
    pass

```

Which of the following functions or classes contains a `CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')` vulnerability?

  A) do_POST
  B) edit_file
  C) Vb4cServer
  D) init_server
Answer: A