On Trials in Which the US Occurs, a Light Precedes

On trials in which the US occurs, a light precedes a tone. On no-US trials, the tone alone is present. Later, the light is changed into a conditioned inhibitor for the US. When light is presented prior to the tone, animals still respond as if the US were forthcoming. The likely conclusion is that

A) the light must be an occasion setter.
B) the light could be an occasion setter, or the light and tone could be forming a configural cue.
C) the observations must be faulty, since a stimulus cannot simultaneously facilitate and suppress responding based on the same US.
D) the light has residual excitation.