/* screenÀÇ width°¡ 640pxÀÌÇÏÀ϶§ ¾Æ·¡¿Í °°Àº ¿É¼ÇÀ» Àû¿ë */
@media screen and (max-width:640px) {
    .selectDiv {width:300px;background:#fff; font-size:20px; color:#000}
}

/* width°¡ ÃÖ¼Ò640 ÃÖ´ë 640px·Îµµ ÁöÁ¤°¡´É */
@media screen and (min-width:640px) and (max-width:640px) {
    .selectDiv {width:600px;background:#fff; font-size:12px; color:#000}
}