What’s the quickest workout to tone lower abs?

I have been doing situps every night for the past couple weeks, and as a result the top part of my abs are firm (think of a 2-pack, ha:/) Anyways, I was wondering what workout is best for the lower abs? I don’t have too much of a problem with it but I would still like to achieve firmer look. Preferrably a workout that is quick.

Thanks in advance.