她大学开始时学英语，但第二年转到了商业研究。

She started studying English at college, but ___switched to Business Studies in her second year.

他刚换了工作。

He's just ___switched jobs.

多年来我一直在 MS-DOS 下创建系统，但两年前我转用了 Unix，从那以后我就用 Unix 工作。

For many years I had created my system under MS-DOS, but two years ago I ___switched and since then I've worked with Unix.