Only after Mary read her composition the second time ___ the spelling mistakes.
I won’t tell the student the answer to the math problem until he ____on it for more than an hour.
Father _____for London on business upon my arrival, so I didn’t see him.
The father as well as his three children ____skating on the frozen river every Sunday afternoon in winter.
Although the causes of cancer _____, we do not yet have any practical way to prevent it.
At this time tomorrow ____over the Atlantic.