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/env python
import csv
import json
import logging
import os
from collections import defaultdict
from html.parser import HTMLParser
from pathlib import Path
from typing import DefaultDict, Dict, List, Optional, Sequence, Tuple

import click
from jinja2 import Template


class HTMLCounter(HTMLParser):
    def __init__(self) -> None:
        super().__init__()
        self.count = 0

    def handle_starttag(self, tag: str, attrs: List[Tuple[str, Optional[str]]]) -> None:
        if tag == "html":
            self.count += 1


def load_template(tmpl_path: str) -> Template:
    with open(tmpl_path, "r", encoding="utf-8") as input_tmpl:
        return Template(input_tmpl.read())


def render_all_content(
    template: Template,
    common_data: Dict[str, str],
    unique_data: Sequence[Dict[str, str]],
    separator: str,
) -> Dict[str, str]:
    addr_to_content: Dict[str, str] = dict()
    mail_defdict: DefaultDict[str, int] = defaultdict(int)
    for data in unique_data:
        data.update(common_data)
        if separator:
            subject = separator.join([data["receiver_email"], data["receiver_name"]])
        else:
            subject = data["receiver_email"]
        mail_defdict[subject] += 1
        # multi-mail
        if mail_defdict[subject] > 1:
            subject = "{}__{:03n}".format(subject, mail_defdict[subject])

        addr_to_content[subject] = template.render(**data)
    return addr_to_content


def export_mails(recv_to_mail: Dict[str, str], output_path: str) -> None:
    for receiver_mail, mail_content in recv_to_mail.items():
        parser = HTMLCounter()
        parser.feed(mail_content)
        if parser.count > 0:
            with open(
                output_path / Path(receiver_mail + ".html"), "w", encoding="utf-8"
            ) as output_html:
                output_html.write(mail_content)
        else:
            with open(
                output_path / Path(receiver_mail + ".txt"), "w", encoding="utf-8"
            ) as output_file:
                output_file.write(mail_content)


@click.command()
@click.argument("template_path", type=click.Path(exists=True))
@click.argument("receiver_data", type=click.Path(exists=True))
@click.option(
    "--separator",
    default="",
    show_default=False,
    help="Separator used for subject suffix. It is disabled with empty string by default.",
)
@click.option(
    "--output_path",
    type=click.Path(exists=False),
    default="mails_to_sent",
    show_default=True,
    help="Output path of rendered mails",
)
@click.option(
    "--unique_csv",
    type=click.Path(exists=True),
    help="Use CSV file to import unique data",
)
def main(
    template_path: str,
    receiver_data: str,
    separator: str,
    output_path: str,
    unique_csv: str,
) -> None:
    """
    Application entry point
    """
    if not os.path.isdir(output_path):
        logging.info('Create directory "%s"', output_path)
        Path(output_path).mkdir(parents=True)

    if unique_csv:
        with open(receiver_data, "r", encoding="utf-8") as input_file:
            data = json.load(input_file)
            common_data = data["common_data"]

        with open(unique_csv, "r", encoding="utf-8-sig") as input_file:
            unique_data = [row for row in csv.DictReader(input_file)]
    else:
        with open(receiver_data, "r", encoding="utf-8") as input_file:
            data = json.load(input_file)
            common_data = data["common_data"]
            unique_data = data["unique_data"]

    template = load_template(template_path)
    recv_to_mail = render_all_content(template, common_data, unique_data, separator)
    export_mails(recv_to_mail, output_path)


# pylint: disable=no-value-for-parameter
if __name__ == "__main__":
    main()
# pylint: enable=no-value-for-parameter

```

Which of the following functions or classes contains a `CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')` vulnerability?

  A) handle_starttag
  B) export_mails
  C) render_all_content
  D) load_template
Answer: D