Elevated phosphine levels in the cloud layers of temperate exoplanets cannot serve as (i) _______ biosignature, despite what their (ii) _______ via next-generation spectroscopy might suggest: phosphine can arise from anaerobic biological metabolism, but it can equally result from lightning-driven photochemical reactions or volcanic outgassing - both strictly abiotic pathways.

Blank (i)	Blank (ii)
A. a provisional	D. refutation
B. a definitive	E. detection
C. a contentious	F. inference