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 os
from flask import render_template,Flask,abort
import json
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy import Column, Integer, String,Text
from sqlalchemy import create_engine
from pymongo import MongoClient


app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql://root:@localhost/test'
db=SQLAlchemy(app)
client=MongoClient('127.0.0.1',27017)
mongodb = client.test
class Category(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True)

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return '<Category %r>' % self.name

class File(db.Model):
    __tablename__='files'

    id = db.Column(db.Integer,primary_key=True)
    title = db.Column(db.String(80))
    created_time = db.Column(db.DateTime)
    category_id = db.Column(db.Integer,db.ForeignKey('category.id'))
    content = db.Column(db.Text)
    category = db.relationship('Category',backref=db.backref('files'))
    def __init__(self,title,created_time,category,content):
        self.title=title
        self.created_time=created_time
        self.category=category
        self.content=content

    #def __repr__(self):
    #    return '<File %r>' % self.title
    
    def add_tag(self,tag_name):
        file_item = mongodb.test.find_one({'file_id': self.id})
        if file_item: 
            tag = file_item['tags']
            print(tag)
            if tag_name not in tag:
                tag.append(tag_name)
            mongodb.test.update_one({'file_id': self.id},{'$set':{'tags':tag}})
        else:
           tag=[tag_name]
           mongodb.test.insert_one({'file_id': self.id,'tags':tag})

    def remove_tag(self,tag_name):
        file_item = mongodb.test.find_one({'file_id': self.id})
        if file_item:
            tags = file_item['tags']
            try:
                tags.remove(tag_name)
            except ValueError:
                return tags
            mongodb.test.update_one({'file_id': self.id},{'$set':{'tags': tags}})
            return tags
        return []


    @property
    def tags(self):
        cat = {'file_id':self.id}
        file_item =  mongodb.test.find_one(cat)
        if file_item:
            return file_item['tags']
        else:
            return []

@app.route('/')
def index():
#    artlist = []
#    engine = create_engine('mysql://root:@localhost/test')
#    artlist =  engine.execute('select * from file').fetchall()
#    for art in artlist:

    return render_template('index.html',artlist=File.query.all())

@app.route('/files/<file_id>')
def file(file_id):

    arts= []
    engine = create_engine('mysql://root:@localhost/test')
    arts =  engine.execute('select f.content,f.created_time,c.name from files f,category c where f.category_id = c.id and f.category_id  = ' + file_id).fetchall()
    


    if arts:
        return render_template('file.html',arts=arts)
    else:
        abort(404)
         
#    select f.content,f.created_time,c.name from file f  join category c on f.category_id = 1 and c.id = f.category_id     
@app.errorhandler(404)
def not_found(error): 
    return render_template('404.html'), 404

         
          
           




if __name__=='__main__':
    app.run()



```

Which of the following functions or classes contains a `CWE-89: Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')` vulnerability?

  A) index
  B) Category
  C) file
  D) tags
Answer: C