But the next thing __ we know was morning.Christmas morning!横线填that 还是which,还是两个都行?

But the next thing __ we know was morning.Christmas morning!横线填that 还是which,还是两个都行?
But the next thing __ we know was morning.Christmas morning!横线填that 还是which,还是两个都行?

But the next thing __ we know was morning.Christmas morning!横线填that 还是which,还是两个都行?
that

当先行词前有any/all/the only/the last等修饰或是先行词为little/all/something/nothing等此时，必须用“that”，有介词时必须用“which”。你所问的题，到底用啥，我也无法确定，但用“that”一定没错。