If there's a 15.5-second time delay between observed lightning and the sound of thunder, approximately how far away is the lightning? Choose one:
A. 5 km
B. 8 km
C. 1.4 miles
D. 2 km

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].

Answerwide-open;

The correct answer is:   [b]:   "physical systems" .