Blimps were once filled with hydrogen, but now helium is used instead. Why is helium a better choice than hydrogen?
a Hydrogen is no longer available in pure form.
b Helium is cheaper than hydrogen.
C Hydrogen is flammable, while helium is not.
d Helium is lighter than hydrogen.

The images show two species of tree frogs in a particular region. the gray tree frogs are adapted to the trees of the woodlands, while the green tree frogs can survive in woodlands or grasslands. the gray frog was abundant in the woodlands. because of climatic changes, most of the trees in the region died, and the woodlands changed to grasslands. in such a case, the . the gray tree frog is closely related to the green tree frog genetically, but they have different mating calls. this suggests that speciation occurred because of .