Experiments of this kind ___inboth the US and Europe well before the Second World War

I didn’t know you were friends with Mr. Smith. — We ______ each other for6 years.

So many people are standing outside the hall. ________ will the concert begin—________ seven o’clock.

Your understanding on the theory is____ satisfactory. You must read more after class.

Your English is very good.—_________