Выберите правильную видо-временную форму глагола и переведите полученные предложения, Заранее спасибо!

1)He (to change)the oil before they started to work.

a)     was changing

b)    had changed

c)     changed

d)    had been changing

2). Recently some new types of farm machinery (to design) . 

a)     has designed

b)    have designed

c)     have been designed

d)    designed

3). Agricultural machinery production in recent years (to be) impressive.

a)     is

b)    was

c)     will be

d)    has been

4. Yesterday I (to repair) this seed drill all day long. 

a) am repairing

b) repaired

c) was repairing

d) will repair

5). I can’t help you right now, I (to check) the hydraulic system pressure. 

a) am checking

b) checked

c) have checked

d) check