A currency system is a system created and used by

Who did the first work in creating vaccines

The rise of advertising in the 1920s. i am good with seeing how advertisements shape the roaring twenties and how it developed a new way of life through modern days. but i am not sure what the details of advertising in the 1920s were. if possible can you send me links (preferrably credential links) to sources that maybe show like the first company to start advertising through the radio, how advertising benefits (statistics graphs), etc.

Which of the following best describes the aztecsa) skilled engineersb) peaceful farmers c) skilled writers d) mountain dwellers