1. Ask a question at a time.
(What are other expressions which are similar to 'at a time'?)
2. John is in his room at the moment.
(What other expressions are similar to 'at the moment'?)
1. I'm not sure there's a good substitute for "at a time." In that sentence, though, I'd change "a" to "one."
2. You could use "right now" instead of "at the moment."
