Complete the sentences with the modal verbs below:
can/can't/could/couldn't /wasn't able to, may/might or have to/had to.

1. People _____ write e-mails until 1990. Since then they have written billions of them.
2. Unfortunately, I really _______ program at all. No-one among my friends is a programmer either.
3. I _________ send my e mail from yahoo mail. There were some problems with the server.
4. The first computers ______ only do simple math formulae like addition and subtraction.
5. With a reference management software on your computer at the end of a lengthy assignment you _______ have more than 100 links.
6. The first programmers ______ face grueling hours of counting and calculating to get operations done.