The statement that is true is that positively charged objects attract negatively charged objects. This is due to a law that states 'like forces attract while unlike forces repel. This same concept applies to magnetism. If you put two similar poles together, for example; if you place two south poles together. You feel a separating force between the two poles. But if you place two opposite poles together they attract each other. Hope i helped. <span />
Yes indeed, that is a true statement. Truer words are seldom written, and I could not agree with you more.
In case you are asking a question, I can kind of see how this might be a question if there is a blank after "... the change in ..." that is to be filled with one or more new words.
If that is the situation, then the blank can be filled with "frequency or wavelength" .
Answer:
b) It is impossible to tell without knowing the masses.
Explanation:
The temperature change of a substance when it receives/gives off a certain amount of heat Q is given by

where
Q is the amount of heat
m is the mass of the substance
Cs is the specific heat capacity of the substance
In this case, we have a hot piece of aluminum in contact with a cold piece of copper: the amount of heat given off by the aluminum is equal to the amount of heat absorbed by the copper, so Q is the same for the two substances. However, we see that the temperature change of the two substances depends on two other factors: the mass, m, and the specific heat, Cs. So, since we know only the specific heat of the two substances, but not their mass, we can't tell which object will experience the greater temperature change.
The focal length, like you said it's the distance between the FOCAL point and the mirror.