To solve the size problem of cell phones, scientists have been researching ___________________ for decades.

A．Tom is always late.
B．Tom will not show up.
C．Tom is usually on time.
D．Tom will be there at 8:30.

A．Find out if a rock-climbing course will be offered.
B．Plan a rock-climbing trip over spring break.
C．Convince the man to take a rock-climbing course with her.
D．Find a place to go rock climbing.