How effective are sit ups for toning the lower abs?

My abs usually stay very toned with very little effort on my part, however, I have been slacking off with my exercises and my lower abs are not as toned as they usually are. The only exercises that I do are sit ups and crunches. I was just wondering exactly how effective sit ups are for the lower abdominal muscles because that seems to be my problem area now.
I don’t need to lose weight, but thanks for the answer, I just need to tone a slightly untoned area on my lower abs.

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