The didn't enter World War 1 until 1917

An education expert is researching teaching methods and wishes to interview teachers from a particular school district. she randomly selects ten schools from the district and interviews all of the teachers at the selected schools. which type of sample did she use in this study? a)systematic sample b)probability sample c)stratified sample d)cluster sample

What period of history refers to the time when people did not have written records

The colors at sunset are usually the result of raindrops