Which of the following statements is/are correct?

a. Chemical analyses have shown that both the Omani copper and Harappan artefacts have traces of nickel, suggesting a common origin. b. Mesopotamian texts refer to copper being brought from a region called Magan, which is possibly a name of Harappa. c. A distinctive type of vessel, a large Harappan jar coated with a thick layer of black clay has been found at Omani sites.