What's mass? a. the sum of the number of protons and neutrons in an atom b. a measure of the amount of matter an object contains c. a measure of an element's attraction for electrons d. the force that gravity exerts upon an object

Mass is a measure of the amount of matter a substance contains.

B. A measure of the amount of matter an object contains

Explanation:

Mass is a measure of resistance which a body offers to its acceleration. Mass is a constant quantity and is the same in every part of the universe.

It also determines the attraction of an object to the gravitational force exerted by earth and other planets that have gravity.

Mass is not the same as weight which is the quantity that gives with which an object is attracted by gravity of a planet.  

lunar landing mission. apollo 11 was the first manned mission to land on the moon. the first steps by humans on another planetary body were taken by neil armstrong and buzz aldrin on july 20, 1969. the astronauts also returned to earth the first samples from another planetary body.

explanation: