  We report the 77Se NMR Knight shift and spin-lattice relaxation studies on
the superconducting state of the ternary iron selenide K0.8Fe2-xSe2 with Tc~30
K. Just below Tc, the Knight shift 77Kn shows an immediate drop, indicating a
singlet pairing. The spin-lattice relaxation rate 1/77T1 decreases rapidly in
the temperature range from TC to Tc/2, which can be fit with an isotropic gap
of \Delta~3.8\pm 0.5 k_BTc. The Hebel-Slichter coherence peak is not observed.
These data give bulk evidence for a strongly coupled superconductivity with
isotropic gaps in K0.8Fe2-xSe2, which is similar to other iron-based high
temperature superconductors. Below Tc/2, the spin-part of the Knight shift
levels off to a constant value (~0.09%), and the spin-lattice relaxation
follows a 1/T1 T^2 behavior, which are not well understood.
