A rapid antigen test strip is used to detect the presence of a viral antigen in a nasal swab sample.

The test strip works as follows:

• The sample is mixed with a buffer and applied to the end of the strip.
• The liquid moves up the strip by capillary action.
• The end of the strip contains mobile detection antibodies conjugated to colored nanoparticles.
• The test zone ($T$) contains immobilized antibodies that bind specifically to the viral antigen.
• The control zone ($C$) contains immobilized antibodies that bind to the free detection antibodies to confirm that the liquid has successfully migrated up the entire strip.

The diagram below shows the test results of four different test strips, labeled A, B, C, and D.

Which test strip represents a valid positive result?