form {
  margin: 0;
  padding: 0;
}
.u-flex {
  display: flex;
  flex-direction: row;
  align-items: center;
}
/* 换行 */
.u-flex-wrap {
  flex-wrap: wrap;
}
/* 不换行 */
.u-flex-nowrap {
  flex-wrap: nowrap;
}
.u-flex {
  display: flex;
  flex-direction: row;
  align-items: center;
}
/* 由于align-items: flex-start在后面，故覆盖了"u-flex"的align-items: center */
.u-col-top {
  align-items: flex-start;
}
/* 垂直居中 */
.u-col-center {
  align-items: center;
}
/* 顶部对齐 */
.u-col-top {
  align-items: flex-start;
}
/* 底部对齐 */
.u-col-bottom {
  align-items: flex-end;
}
/* 左边对齐 */
.u-row-left {
  justify-content: flex-start;
}
/* 水平居中 */
.u-row-center {
  justify-content: center;
}
/* 右边对齐 */
.u-row-right {
  justify-content: flex-end;
}
/* 水平两端对齐，项目之间的间隔都相等 */
.u-row-between {
  justify-content: space-between;
}
/* 水平每个项目两侧的间隔相等，所以项目之间的间隔比项目与父元素两边的间隔大一倍 */
.u-row-around {
  justify-content: space-around;
}