N-COUNT 杂质;不纯物质 

 Impurities are substances that are present in small quantities in another substance and make it dirty or of an unacceptable quality.

and make it dirty or of an unacceptable quality.是第二个省略关系代词的定语从句还是与 Impurities are substances that are present in small quantities in another substance 并列的主句

Press in the tap handle，and turn anti-clockwise to full-on，or ，in the case of the oven burner，open the oven door，push the Regulo oven tap inward and turn to the selected Mark.

in the case of the oven burner修饰

open the oven door，push the Regulo oven tap inward and turn to the selected Mark.

这三个并列的主句吗？