Vì sao một quốc gia không nên sử dụng đồng thời nhiều loại "tiền"(như đồng nội tệ, ngoại tệ, vàng,..)làm phương tiện thanh toán

Pj conducts an experimental study on the effects of soft music during high stakes science testing. he randomly assigns students at the school. in one condition he does not provide music for testing while in the other group he does provide the music. he administers a pretest at the beginning of the year and a posttest at the end of the year. pj’s design is best described as a: a. static group comparisonb. pretest-posttest control group designc. control-group-time-series designd. nonrandomized control-group pretest-posttest design