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17.A friend of my cannot connect her phone
A. A            B.of             C.my             D.Her
18. What __________ when his mother came?
A. were you dong              B.was you doing            C.are you doing
19.Air ________________, together with littering,is causing many problems in our cities today
A.pollute             B.pollution             C.polluting                D.polluted
20.We cleaned up everythong at the campsite ______________ we left
A.until               B.before             C.while                 D.after