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
from flask import render_template, flash, redirect, url_for, request
from app import app
from app.forms import SubmissionForm, LoginForm, RegistrationForm, EditForm
from app.models import Post, User
from app import db
from flask_login import login_user, logout_user, current_user, login_required


@app.route('/')
@app.route('/index')
def index():
    print("hello world")
    if current_user.is_authenticated:
        current_posts = Post.query.all()
        return render_template('index.html', title='Home', posts=current_posts, logged_in = True)
    return render_template('notloggedin.html')

@app.route('/posts/<postid>')
def showpost(postid):
    if current_user.is_authenticated:
        current_post = Post.query.filter_by(id=postid)[0]
        author = current_post.author #User.query.filter_by(id=current_post.user_id).first_or_404()
        return render_template('showpost.html', title='Show Post', postid=postid, post=current_post, author=author)
    return render_template('notloggedin.html')

  


@app.route('/edit/<postid>', methods=['GET', 'POST'])
def editpage(postid):
    curr_post = Post.query.filter_by(id=postid)[0]
    if current_user.is_authenticated:
            form = EditForm()
            if request.method == 'POST':
                flash('Topic has been submitted.')
                curr_post.body = form.text_2.data
                db.session.commit()
                fil = postid + ".txt"
                f = open(fil, 'w')
                f.write(curr_post.body)  # python will convert \n to os.linesep
                f.close()  # you can omit in most cases as the destructor will call it
                return redirect(url_for('index'))
            else:
                form.text_2.data = curr_post.body
                return render_template('edit.html',  title='Edit New Entry', form=form)
    return render_template('notloggedin.html')


@app.route('/submit', methods=['GET', 'POST'])
def submitpage():
    if current_user.is_authenticated:
        #print(current_user)
        form = SubmissionForm()
        if form.validate_on_submit():
            flash('Topic {} has been submitted.'.format(
                form.title.data))
            new_title = form.title.data
            new_text = form.text.data
            user_id = current_user.id
            p = Post(title=new_title,body=new_text,user_id=user_id)
            #print(p.user_id)
            db.session.add(p)
            db.session.commit()
            return redirect(url_for('index'))
        return render_template('submit.html',  title='Submit New Entry', form=form)
    return render_template('notloggedin.html')


@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user is None or not user.check_password(form.password.data):
            flash('Invalid username or password')
            return redirect(url_for('login'))
        login_user(user, remember=form.remember_me.data)
        next_page = request.args.get('next')
        if not next_page or url_parse(next_page).netloc != '':
            next_page = url_for('index')
        return redirect(next_page)
    return render_template('login.html', title='Sign In', form=form)


@app.route('/logout')
def logout():
    logout_user()
    return redirect(url_for('index'))


@app.route('/register', methods=['GET', 'POST'])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = RegistrationForm()
    if form.validate_on_submit():
        user = User(username=form.username.data, email=form.email.data)
        user.set_password(form.password.data)
        db.session.add(user)
        db.session.commit()
        flash('Congratulations, you are now a registered user!')
        return redirect(url_for('login'))
    return render_template('register.html', title='Register', form=form)


@app.route('/user/<username>')
@login_required
def user(username):
    user = User.query.filter_by(username=username).first_or_404()
    posts = user.posts
    return render_template('user.html', user=user, posts=posts)



@app.errorhandler(404)
def not_found_error(error):
    return render_template('404.html'), 404

@app.errorhandler(500)
def internal_error(error):
    db.session.rollback()
    return render_template('500.html'), 500

```

Which of the following functions or classes contains a `CWE-601: URL Redirection to Untrusted Site ('Open Redirect')` vulnerability?

  A) submitpage
  B) login
  C) user
  D) register
Answer: B