20. The flu is believed________by viruses.
. .—The last one________pays the meal. —Agreed!
. I have no idea________is better, so I took of them.
. —Who did you spend last weekend with? —________.
. —Mike, what did our monitor say just now? —Every boy and every girl as well as teachers who________to visit the museum asked to be at the school gate before 6:30 in the morning.
. .—Can those ________at the back of the classroom hear me? —No problem.