The conventional view    that the Mars’ atmosphere is very poor because the planet lacks a magnetic field.

We must ________ our task, however hard it is.

At the opening ceremony, the chairman ______ a speech to welcome the guests from more than twenty centuries.

Happy birthday, Alice? So you have _______ twenty-one already?

-- I’ m surprised to hear that Sue and Paul have __  .
----So am I. They seemed very happy together when I last saw them.

Everyone of us has many responsibilities ______us．So we must work hard．

一 Mom, guess what? I’ve won the 1st prize!
一 This           a celebration! Let’s go to prepare a party．

The level of the river ___ 20 centimeters over the security line and ____must be taken to control the flood.

It’s two years since they ____ each other， and they are going to get married next month.

Such animals as dinosaurs died out because they couldn’t ____ to the sudden change of the climate.