Our friends were picked up by our parents.
which changes the sentence from passive to active voice without changing its meaning?

This sentence is currently in passive voice. In active voice, it would be:

"Our parents picked up our friends"

To change a sentence from passive to active voice you would switch around the subjects. In passive voice, the subject (in this case "parents") is being acted upon. In active voice, the parents need to act upon "friends."

after the signature of the treaty of versailles in june 1919, germany lost nearly 25,000 square miles and a population of 7 million people. these territories included eupen-malmedy area and moresnet which were ceded to belgium. regarding relations with france, germany ceded the sovereignty of the saar coal mines to this country, as well as the provinces of alsace-lorraine.

explanation: