#cars {
width: 100%;
padding: 12px 20px;
margin: 8px 0;
display: inline-block;
border: 1px solid #ccc;
border-radius: 4px;
box-sizing: border-box;
}

var cars = document.getElementById("cars");
cars.addEventListener("change", function() {
var selectedCar = cars.options[cars.selectedIndex].value;
alert("You selected " + selectedCar);
});