Only one sine marker closes the selected spacing, order, and wavelength. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Low sine marker undershoots the target
Spacing 10 m times sine marker 1/5 gives 2 m. Order 2 times wavelength 2 m gives 4 m, so this row is rejected.
10m⋅51=2mmλ=2⋅2m=4mA=0
Middle sine marker closes exactly
Spacing 10 m times sine marker 2/5 gives 4 m. Order 2 times wavelength 2 m gives 4 m, so this row is accepted.
10m⋅52=4mmλ=2⋅2m=4mA=1
High sine marker overshoots the target
Spacing 10 m times sine marker 3/5 gives 6 m. Order 2 times wavelength 2 m gives 4 m, so this row is rejected.