1 It is hard to imagine how big a billion is. A billion is 1 000 000 000 (or one thousand
million). There are 1 000 millimetres in 1 metre and 1 000 metres in 1 kilometre.
How many millimetres are in 1 kilometre?

of those who start drinking before they're 15 will develop alcohol dependence

the electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, x-rays, and gamma-rays.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?

voluntary muscles are controlled by the cerebellum.