Nucleus X decays radioactively to nucleus Y, which is itself radioactive. The half-life of X is 8h, and the half-life of Y is 2h. A sample initially contains only nuclei X. After 8h, what is the ratio NY/NX of the number of Y nuclei to the number of remaining X nuclei?
a) 0.44
b) 1.0
c) 0.29
d) 0.18