Antibody levels below protective threshold indicate infection

Steps:

• Examine each graph for antibody level trends relative to the PAL line over time.
• Identify groups where levels start below PAL and fail to rise sufficiently or sustain protection.
• Compare peak and duration of antibody response across A, B, C, D.
• Select the group showing inadequate response leading to viral effects.

Why B is correct:

• Group B's antibody levels remain below PAL throughout, per immunology definition where sub-protective levels allow viral replication and symptoms.

Why the others are wrong:

• A: Levels exceed PAL quickly, providing immunity.
• C: Sustained elevation above PAL prevents infection.
• D: Rapid rise to and above PAL confers protection.

Final answer: B