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
import requests
import re
import pandas as pd
from bs4 import BeautifulSoup
from fake_useragent import UserAgent
import time
import pprint


# index_url = "https://www.bookmarkearth.com/page?currentPage=1"


class BookmarkSpider(object):
    def __init__(self):
        self.ua = UserAgent()
        self.index_url = "https://www.bookmarkearth.com/page?currentPage=1"
        self.headers={
        'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36 QIHU 360SE'
    }

    def get_real_url(self, url="http://show.bookmarkearth.com/view/799?folderId=28178"):
        print(url)
        try:
            res = requests.get(url,timeout=100,headers=self.headers)
            soup = BeautifulSoup(res.text, 'lxml')
            
            tags=soup.find(attrs={"class": "nav-left left-menu "})
     
            if tags is not None:
                real_url = tags.find('li').a.get("data-url")
                real_url_desc = tags.find('li').a.get("data-name")
            else:
                real_url, real_url_desc=['','']
            print(real_url_desc)
            return real_url, real_url_desc
        except Exception as e:
            print(e)
            time.sleep(5)
            return self.get_real_url(url)

    def main(self):
        pagenums = self.get_pagenums()
        # pagenums = 1
        print("共有【%s】 个页面需要解析" % pagenums)
        url_lst = ["https://www.bookmarkearth.com/page?currentPage=%s" % (i + 1) for i in range(pagenums)]
        bookmarks_lst = []
        for url in url_lst:   
            soup = self.start_requests(url)
            bookmarks = self.parse_bookmark(soup)
            print("此页面共获取到%s个书签" % len(bookmarks))
            print('获取到的前五个书签名称为：%s' % bookmarks[:5])
            bookmarks_lst.extend(bookmarks)
            print("\n" * 2)
        df = pd.DataFrame(bookmarks_lst, columns=['书签名称', '链接', '简介'])
        df = self.bookmarks_clean(df)
        df.to_excel("书签地球%s页_%s.xlsx" % (pagenums, time.strftime("%Y%m%d%H%M%S")), index=False)

    def start_requests(self, url):
        print("【请求页面】： %s" % url)
        try:
            res = requests.get(url, headers=self.headers,timeout=100)
            return BeautifulSoup(res.content, 'lxml')
        except Exception as e:
            print(e)
            time.sleep(5)
            return self.start_requests(url)
        
        

    def parse_bookmark(self, soup):
        bookmark_lst = soup.find_all(rel='noreferrer nofollow')
        url_pat = r"[a-zA-z]+://[^\s]*"
        bookmarks = []
        for bookmark in bookmark_lst:
            name = bookmark.text
            url = bookmark.attrs.get('href')
            real_url, real_url_desc = self.get_real_url(url)
            if re.match(url_pat, url) is not None:
                bookmarks.append([name, real_url, real_url_desc])
        return bookmarks

    def get_pagenums(self):
        soup = self.start_requests(self.index_url)
        pagenum_texts = soup.find(attrs={'class': 'white-space'})

        # '当前 1/117页'
        pagenum = pagenum_texts.text.split("/")[-1].strip("页")
        return int(pagenum)

    def bookmarks_clean(self, df):
        urls_count = df['链接'].value_counts()
        urls_count_dict = urls_count.to_dict()
        df['收藏人数'] = df['链接'].apply(lambda x: urls_count_dict.get(x))
        df.sort_values(by=['收藏人数'], ascending=False, inplace=True)
        df.drop_duplicates(['链接'], inplace=True)
        return df


if __name__ == "__main__":
    bookmarkspider = BookmarkSpider()
    bookmarkspider.main()

```

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

  A) __init__
  B) start_requests
  C) parse_bookmark
  D) get_real_url
Answer: C