As we know, modal verbs "could" & "can't" can be used to guess something or state an ability.
Eg: She could be at home now (=It is possible that she is at home now. The chance is 50%)
She can't be at home (=I am quite sure that she is not at home now. The chance is 85-90%
I could swim (= I had an ability to swim in the past)
I can't swim (=I don't have an ability to swim now)
Now, see this conversation int "The social network"
Mark: Did you know there are more people with genius IQs living in China than there are people of any kind living in the United States?
Mark'girlfriend: That can't possibly be true.
Mark: It is.
Mark's girlfriend: What would account for that? Mark: Well, first, an awful lot of people live in China, but here's my question. How do you distinguish yourself in a population of people who all got 1600 on their SATs?
Mark's girlfriend: - I didn't know they take SATs in China.
Mark:-They don't. I wasn't talking about China anymore, I was talking about me.
Mark's girlfriend: You got a 1600?
Mark: Yes. I could sing in an a Capella group, but I can't sing.
Mark's girlfriend: Does that mean you actually got nothing wrong?
Mark: I could row crew or invent a $25 PC.
My question is that:
The "could" & "can't" the sentence "I could sing in an a Capella group, but I can't sing." express a guess or an ability?
Does "I could sing in an a Capella group" mean "I guess I have 50% chance of singing in an a Capella group now" or "I had an ability to sing in an a Capella group in the past"
Does "but I can't sing." mean "I am quite sure that I have 90% chance of not singing in an a Capella group now" or "I don't have an ability to in an a Capella group now"