The core of the earth is believed to be composed of iron and nickel.
a. true
b. false

Yes, it is true that the core of the Earth is believed to be composed of iron and nickel, since the inner core is believed to be made of iron, with the outer core being made of nickel. 

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.