Astronomer: Most stars are born in groups of thousands, each star in a group forming from the same parent cloud of gas. Each cloud has a unique, homogeneous chemical composition. Ttherefore, whenever two stars have the same chemical composition as each other, they must have originated from the same cloud of gas. Which of the following, if true, would most strengthen the astronomer's argument? ['Many stars in vastly different parts of the universe are quite similar in their chemical compositions.', 'Clouds of gas of similar or identical chemical composition may be remote from each other.', 'In some groups of stars, not every star originated from the same parent cloud of gas.', 'Whenever a star forms, it inherits the chemical composition of its parent cloud of gas.']