How has the Sahara Desert contributed to nomadic herding in northern Africa?

The chart shows four articles read by the students in a science class

There are 25 elements found in living things. how many of these elements are found in some organisms but not all? 1 , 6 , 19 , 25

The primary purpose of a certificate of confidentiality is to: allow law enforcement to investigate abuse cases. protect researchers from disclosing conflicts of interest. prevent subjects from knowing the purpose of a study. protect identifiable research information from compelled disclosure.