Why should there be only one variable in an experiment

1. read the following excerpt from franklin d. roosevelt's "fireside chat 19":

on the other side of the picture, we must learn also to know that guerilla warfare against the germans in, let us say serbia or norway, us; that a successful russian offensive against the germans us; and that british successes on land or sea in any part of the world strengthen our hands.

remember always that germany and italy, regardless of any formal declaration of war, consider themselves at war with the united states at this moment just as much as they consider themselves at war with britain or russia. and germany puts all the other republics of the americas into the same category of enemies. the people of our sister republics of this hemisphere can be honored by that fact.  

the true goal we seek is far above and beyond the ugly field of battle. when we resort to force, as now we must, we are determined that this force shall be directed toward ultimate good as well as against immediate evil. we americans are not destroyers — we are builders.

analyze the effectiveness of this excerpt. how does it function within the speech as a whole? how does the structure of the excerpt to convey the speech's purpose and key points? support your argument with evidence from the speech. (10 points)

malvolio only loves olivia because he thinks if they get married to olivia he will have a noble title, people to serve him, and fine clothes to wear. also he thinks if they get married he will have many expensive things, and he thinks he will be able to order toby around. malvolio only wants to be noble and boss people around he dose not really love olivia. but viola love for duke orsino is real she really loves orsino.

i think he will fail because he tried to get her to love him and she said oh, you are sick of self-love, malvolio. there is no slander in an allowed fool. so i think she knows what malvolio is trying to do. she knows that he only wants to marry her because he wants to be noble and boss people around so that it why i think she said oh, you are sick of self-love, malvolio. there is no slander in an allowed fool because she new he was selfish.