John waited at the bus-stop for nearly half an hour the bus finally arrived .
More than a dozen students in that school _______ abroad to study medicine last year.
----Why did you leave that position? ----I ______ a better position at IBM.
Some experts demanded that children _______ time to sleep and play.
The farmer used wood to build a houseto store grain.
According to the periodic table,still some elements undiscovered.