Scientists are trying their best to_______ ways to treat the terrible disease caused by Ebola(埃博拉).
—________ does it take you to walk to school? —About 20 minutes.
—Do you go there by bike ________ by bus? —By bike.
What do you ________ the workers in the factory?
—________ do you usually go to school,Simon? —By bike.
Do you know ________?