After Jack had sent some e-mails, he _______ working on his project.

With the rapid development of medical science, we ________ more new drugs used in the treatment of cancer. 

More than a dozen students in that school ________ abroad for further study every year.

  When I ______home, I caught a thief stealing from a passer-by.

I suggest you delete the useless files on the computer which _______ too much space.