There’s a high probability that a 100m asteroid has already been spotted ahead of time. What could we do about it? Not much.

OTOH, there haven’t been any such strikes in -recorded- history. We can rest assured that, for the living, the odds of not experiencing one are very good. Also, the Earth is 3/4 water, that’s where they’re likely to strike. People on the shores nearby would be in the most danger… but with warning time.