19. most of the information about earth’s interior was obtained by studying (1 point) a. earthquake waves b. rocks of the ocean crust-my answer    c. meteorites d. rocks in deep wells 20. the continental crust has the average composition of (1 point a. gneiss b. granite c. basalt d. limestone-my answer    21. the greatest concentration of metals occurs in earth’s (1 point) a. oceanic crust b. continental crust c. core -my answer    d. mantleam i right?

19. is A 20. is B 21. is C 

The correct answer should be brand c.

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].