Should I respond to a girl that just says "mhm" all the time? if so how?

I've been texting/talking to this girl for the past couple of weeks, when we first met she seemed really into me but whenever we talk or text on the phone she just always says "mhm" should/how should I respond?
Should I respond to a girl that just says "mhm" all the time? if so how?
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