Among the 15-year-old children, some ____ studying in university while others are still learning to read and

write. [ ]

A. should have been

B. would have been

C. can be

D. must be

根据《票据法》的规定，付款人承兑汇票，不得附有条件，承兑附有条件的，所附条件不具备票据上的效力。( )