Nina has read the newspaper already.

=> (a)  

They have seen these movies yet.

=> (a)  

I have signed the contract.

=> (a)  

These men have bought the tickets.

=> (a)  

Vicky has found her hat.

=> (a)  

I have done my exercise for three days.

=> (a)  

Tom and Jenny have bought this teddy bear in that shop

=> (a)  

These newspapers have been bought by Nina.

=> (a)  

Computer science has been studied by my brother since June.

=> (a)