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
"""

@Author  : dilless
@Time    : 2018/6/25 22:46
@File    : paper_download.py
"""
import os
import re
from datetime import datetime

import MySQLdb
from selenium import webdriver
from selenium.common.exceptions import NoSuchElementException


class MySQLAccess(object):
    def __init__(self):
        self.conn = MySQLdb.connect('127.0.0.1', 'root', 'root', 'xueshu_spider', charset='utf8', use_unicode=True)
        self.cursor = self.conn.cursor()

    def update_file_path(self, file_path, url):
        sql = "update paper set file_path = '{0}' where url = '{1}'".format(file_path, url)
        try:
            self.cursor.execute(sql)
            self.conn.commit()
        except Exception as e:
            print(e)

    def get_urls(self):
        """

        :return: urls tuple in tuple
        """
        sql = "select url from paper where is_down = 0"
        try:
            self.cursor.execute(sql)
            urls = self.cursor.fetchall()
            return urls
        except Exception as e:
            print(e)

    def update_is_down_status(self, url):
        sql = "update paper set is_down = 1 where url = '{0}'".format(url)
        try:
            self.cursor.execute(sql)
            self.conn.commit()
        except Exception as e:
            print(e)


class DownloadPaper():
    def __init__(self):
        options = webdriver.ChromeOptions()
        prefs = {'profile.default_content_settings.popups': 0,
                 'download.default_directory': 'D:\Files\PythonProject\XueshuSpider\down_papers'}
        options.add_experimental_option('prefs', prefs)
        self.browser = webdriver.Chrome(executable_path='D:\Files\PythonProject\chromedriver.exe',
                                        chrome_options=options)
        self.browser.implicitly_wait(5)

    def download_paper(self, url):
        self.browser.get(url)
        try:
            if re.match(r'^(http://kns.cnki.net).*', url):
                # 知网期刊
                self.browser.find_element_by_css_selector('#pdfDown').click()
            if re.match(r'^(http://http://cdmd.cnki.com.cn).*', url):
                # 知网空间
                self.browser.find_element_by_css_selector('#ty_caj > a:nth-child(1)').click()
            if re.match(r'^(http://cpfd.cnki.com.cn).*', url):
                # cpfd知网
                self.browser.find_element_by_css_selector('#ty_pdf > a:nth-child(1)').click()
            if re.match(r'^(http://www.wanfangdata.com.cn).*', url):
                # 万方
                self.browser.find_element_by_css_selector('#ddownb').click()
        except NoSuchElementException as e:
            current_path = os.path.dirname(__file__)
            project_path = os.path.join(current_path, '../..')
            down_path = os.path.abspath(os.path.join(project_path, 'down_papers'))
            file_path = os.path.join(down_path, 'down_error_log.txt')
            with open(file_path, 'w+', encoding='utf-8') as error_file:
                error_file.write(str(datetime.now()) + ': ' + url + " -- don't provide download.")
                error_file.write('\r\n')


if __name__ == '__main__':
    db = MySQLAccess()
    down = DownloadPaper()
    urls = db.get_urls()
    for url in urls:
        page_url = url[0]
        down.download_paper(page_url)
        db.update_is_down_status(page_url)

```

Which of the following functions or classes contains a `CWE-20: Improper Input Validation` vulnerability?

  A) download_paper
  B) get_urls
  C) update_is_down_status
  D) __init__
Answer: A