Symmetry in Square Pattern Reflection

In a classroom, a teacher draws a large square on the board that represents the classroom's floor plan. This square is divided into four equal smaller squares by drawing a vertical line and a horizontal line through its center. The teacher then shades two diagonal squares in opposite corners, creating a pattern.

If the teacher reflects this pattern over the vertical line, which squares will remain shaded?

Consider the arrangement of the squares and their respective positions before answering. Each square is labeled as follows:

Top Left: Square A
Top Right: Square B
Bottom Left: Square C
Bottom Right: Square D