=== Model: grok-4.6-direct | Task: bench:gpqa_diamond | Rep: 1 ===
=== Score: 5/5 | Wall: 49.838s | Cost: $0.020058 ===
=== Pass: True | Finish: stop | In-reasoning-only: False ===

--- JUDGE ---
gpqa_diamond

--- RESPONSE ---
The mutant phenotypes indicate that G2 is essential for any resistance (complete loss in \(g2\) and all \(g2\) double mutants). This is the pattern expected if G2 is a transcription factor required for expression of the other resistance genes (and possibly additional untested targets).  

G1 and G3 each make a partial, independent contribution (single-mutant resistances of 75 % and 50 %). Their double-mutant resistance collapses to 10 %, far below either single mutant; this synergistic enhancement is the genetic signature of functional redundancy.  

The remaining options can be excluded on both logical and semantic grounds:  
- a shared promoter cannot be inferred from knockout phenotypes,  
- pleiotropy cannot be assessed from a single trait,  
- G1 cannot be the upstream transcription factor because its single-mutant phenotype is the weakest, not the strongest.  

Thus only statement A is consistent with the data.  

The answer is (A)