Numbers are expressed in scientific notation using exponents

By using exponents, we can reformat numbers. This can be helpful, in much the same way that it's helpful (that is, it's easier) to write "twelve trillion" rather than 12,000,000,000,000., or "thirty nanometers" rather than "0.00000003 meters" very large or very small numbers, it is sometimes simpler to use "scientific notation" (so called, because scientists often deal with very large and very small numbers).The format for writing a number in scientific notation is fairly simple: (first digit of the number) followed by (the decimal point) and then (all the rest of the digits of the number), times (10 to an appropriate power).

answer: b

coz red blood cells marrow and circulates the body for 120 days.