问重新加载页面10次后，Express停止工作

EN

var bcrypt = require('bcrypt');
var pool = require('../database.js').pool;


module.exports = function(app) {
    app.get('/learn', function(req, res, next) {
        var query = 'SELECT * FROM questions INNER JOIN answers ON questions.questionID = answers.questionID';
        pool.getConnection(function(err, connection) {
            connection.query(query, function(err, rows) {
                if (err) {
                    throw err;
                }

                var data = {
                    name: req.session.name,
                    problems: rows,
                };

                res.render('learn.html', data);
            });
        });
    });

    app.post('/learn/checkAnswer', function(req, res) {
        //get posted form data
        var questionID = req.body.questionID;
        var selectedAnswer = req.body.selectedAnswer;

        //query database
        pool.getConnection(function(err, connection) {
            var query = connection.query('SELECT correctAnswer FROM questions WHERE questionID = ?', questionID, function(err, rows) {
                res.send({
                    correctAnswer: rows[0].correctAnswer
                });
            });
        });
    });
};