.study_inner:nth-child(1) .img {
    background: url(../images/thesis/01-1.jpg) no-repeat center/cover;
}

.study_inner:nth-child(2) .img {
    background: url(../images/thesis/02-1.jpg) no-repeat center/cover;
}

.study_inner:nth-child(3) .img {
    background: url(../images/thesis/03-1.jpg) no-repeat center/cover;
}

.study_inner:nth-child(4) .img {
    background: url(../images/thesis/04-1.jpg) no-repeat center/cover;
}

.study_inner:nth-child(1) .txt {
    background: url(../images/thesis/01-2.jpg) no-repeat center/cover;
}

.study_inner:nth-child(2) .txt {
    background: url(../images/thesis/02-2.jpg) no-repeat center/cover;
}

.study_inner:nth-child(3) .txt {
    background: url(../images/thesis/03-2.jpg) no-repeat center/cover;
}

.study_inner:nth-child(4) .txt {
    background: url(../images/thesis/04-2.jpg) no-repeat center/cover;
}

.study_inner:nth-child(1) .txt:hover,
.study_inner:nth-child(1) .txt:focus {
    background-image: url(../images/thesis/01-3.jpg);
}

.study_inner:nth-child(2) .txt:hover,
.study_inner:nth-child(2) .txt:focus {
    background-image: url(../images/thesis/02-3.jpg);
}

.study_inner:nth-child(3) .txt:hover,
.study_inner:nth-child(3) .txt:focus {
    background-image: url(../images/thesis/03-3.jpg);
}

.study_inner:nth-child(4) .txt:hover,
.study_inner:nth-child(4) .txt:focus {
    background-image: url(../images/thesis/04-3.jpg);
}