27. Regarding the "gene-for-gene hypothesis" in the plant immune system, which of the following conditions must be met for a plant to successfully recognize and trigger a defense response against a specific pathogen?
(A) The plant has a dominant resistance (R) gene, and the pathogen has the corresponding avirulence (Avr) effector gene.
(B) The plant lacks an R gene, and the pathogen has an Avr gene.
(C) The plant has an R gene, and the pathogen lacks an Avr gene.
(D) Neither the plant nor the pathogen expresses any defense or pathogenic genes.