三、綜合測驗(克漏字) 
(A) In 1970, a scientist __21__ Spencer Silver was doing experiments in the 3M research laboratories trying to develop a powerful adhesive. He made a new adhesive, but somehow it was much weaker than __22__ 3M already manufactured. Objects stuck together with it could easily be parted. It should have been very strong, but it turned out to be very weak. Nobody knew what to do with it, but Silver just kept it. Four years later, when his colleague Arthur Fry was singing in a church __23__, he used markers to keep his place in the book of hymns, but they kept falling out. __24__ Silver's weak adhesive, Fry coated his markers with it and it was a big success: they stayed in place, yet lifted off without damaging the pages. That was __25__ Post-it Notes were invented!
【題組】21.
(A) calls
(B) called
(C) calling
(D) to call