Which of the following is not a process that forms metallic ore?
a. placer deposits
b. fossil compaction
c. hydro-thermal solutions
d. igneous process

Fossil compaction is not a process which forms metallic ore whereas the other three ie placer processes, hydrothermal fluids and igneous processes all can help to form metallic ores. Placer does it by stream transport and separation of the heavy gold, and the fluids can carry metals and deposit them in the fractures in the solid rock and igneous processes like fractional crystallization can form a concentration of metallic ores also. 

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].