I listened to Dr．Wilson's lecture about the protection of wild animals, but I was unable to grasp its key     .

You should follow________, and take medince three times a day.

Betty has a very tight _____ this weekend so she has turned down the dinner invitation.

We look forward to the day when the motorcar has been replaced by some less dangerous ____ of transport.

Johnson is an honest businessman. Our company and his have had a lot of ______ in the past few years.