— What are the _______ doing there?
— They are busy _______ their science experiment.
| A．Boys students; to do | B．Boy students; to do |
| C．Boys students; doing | D．Boy students; doing |
— What are the _______ doing there?
— They are busy _______ their science experiment.
| A．Boys students; to do | B．Boy students; to do |
| C．Boys students; doing | D．Boy students; doing |
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