A diagnostic lateral flow test strip is used to detect the presence of a specific viral antigen in a blood sample.

The test strip functions as follows:

• The sample is applied to the sample pad and migrates up the strip by capillary action.
• The conjugation pad contains mobile monoclonal antibodies conjugated to colored latex beads.
• The test zone ($T$) contains immobilized monoclonal antibodies specific to the viral antigen.
• The control zone ($C$) contains immobilized antibodies designed to bind to the mobile conjugated antibodies, confirming the sample has successfully traversed the entire strip.

The diagram below shows the test results of four different strips, labeled 1, 2, 3, and 4.

Which test strip represents a valid negative result?