Can my parents get B-1 visa?

Back in 2007, in order to avoid the hassle of applying for the visa each time they visit USA, I applied green card for my parents. They got the green card in the summer of 2007. They visited USA a couple of times after that, each time stay for about half a year. They stayed in China for about 1 year after each visit. It has been 1.5 years since their last visit. They just can't fulfill the requirement that they have to stay in USA at least 6 month every year. So they decide to give up the green card.

They plan to come to visit us again this spring (this will be almost 2 years since last visit). Can they get B-1 visa after they give up their green card? What is the correct procedure to go through all of this?

Thanks in advance.