1. I hard, so I felt very tired.
a) had worked - if you want to emphasise that the job had been finished before you felt tired
c) had been working - if you want to say that performing some work made you tired
2. I my money into real estate.
b) have invested - if you mean that it's going give you profits
a) invested - if it happened some time ago and there's no connections to present situations
3. We alreadythe goods when your telegram arrived.
b) had shipped
4. The invoice by the end of the week so we sent them a strong reminder.
c) hadn't arrived
5. I nothing on the Internet this month.
a) have bought
6. This strategy seems to be good. Sales of our products for some time.
b) have been increasing
Our son is as old as yours
My English is as good as his
His hair is longer than hers
Our dog is more beautiful than theirs
Their scores are higher than ours