  mul: document.getElementById('op-mul'),
          div: document.getElementById('op-div'),
        },
        startBtn: document.getElementById('startBtn'),
        stopBtn: document.getElementById('stopBtn'),
        resetBtn: document.getElementById('resetBtn'),
        modeSummary: document.getElementById('modeSummary'),
        timeLeft: document.getElementById('timeLeft'),
        question: document.getElementById('question'),
        answer: document.getElementById('answer'),
        submitBtn: docum