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the waiter had to chance the plates several times, ........?( had he\ hasn't he\didn't he\did he)
There should be a reason ......... everything.(of\to\for\at)

the waiter had to chance the plates several times, ........?( had he\ hasn't he\didn't he\did he)There should be a reason ......... everything.(of\to\for\at)didn't hefor