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 json
import pymongo
from flask import Flask, request
from bson import ObjectId


replica1 = 'mongod-0.mongodb-service.default.svc.cluster.local:27017'
replica2 = 'mongod-1.mongodb-service.default.svc.cluster.local:27017'
replica3 = 'mongod-2.mongodb-service.default.svc.cluster.local:27017'
uri = 'mongodb://main_admin:abc123@{},{},{}/?replicaSet=MainRepSet'.format(replica1, replica2, replica3)


client = pymongo.MongoClient(uri)
db = client.test
agents_db = db.agents

app = Flask(__name__)


class JSONEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, ObjectId):
            return str(o)
        return json.JSONEncoder.default(self, o)


@app.route('/agents', methods=['GET', 'POST'])
def agents():
    if request.method == 'POST':
        agent = request.get_json(force=True)
        try:
            agent_id = agents_db.insert_one(dict(agent)).inserted_id
        except pymongo.errors.DuplicateKeyError:
            return json.dumps({
                'result': False,
                'error_message': 'Agent with this codename already exists',
            })
        except TypeError:
            return json.dumps({
                'result': False,
                'error_message': 'Incorrect input data',
            })

        return json.dumps({
            'result': True,
            'agent_id': JSONEncoder().encode(agent_id),
            'error_message': None,
        })
    else:
        return json.dumps({
            'result': True,
            'data': [],
        })


@app.route('/agents/<string:codename>', methods=['GET', 'POST'])
def get_agent(codename):
    if request.method == 'POST':
        agent = request.get_json(force=True)
        if db.agents.find_and_modify(query={'codename':codename}, update={"$set": dict(agent)}, upsert=False, full_response= True):
            return json.dumps({'result': True})
        else:
            return json.dumps({
                'result': False,
                'error_message': "Couldn't find agent {}".format(codename),
            })

    else:
        agent = db.agents.find_one({ 
            'codename': codename
        })
        if agent is not None:
            return json.dumps({
                'result': True,
                'agent': JSONEncoder().encode(agent),
            })
        else:
            return json.dumps({
                'result': False,
                'error_message': "Cannot find agent".format(codename),
            })


if __name__ == '__main__':
    app.run(host='0.0.0.0')

```

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

  A) default
  B) get_agent
  C) agents
  D) JSONEncoder
Answer: B