All except C are correct.
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CalifJimthe more idiomatic practice is to use the -ing form before the verb and the to form after the verb when there is a choiceThat is what I have encountered most; to a point where I'm having trouble accepting g). I guess it's just because it's completely new to me.
Henry74There's still something that I don't understand, but I'm not even sure what it is at the moment.Join the club! This is not an easy subject for any of us.
Henry74g2) It's useless to ask Sue to help.Here is a useful resource on the variety of ways we use infinitives.