The conversion of farmable or habitable land to desert, as by a change in climate or destructive land use, is known as
a. transformation.
b. global warming.
c. desertification.
d. greenhouse effect.

Desertification (C) is the conversion of farmable or habitable land to desert, as by a change in climate or destructive use. 

Ifound the exercise on the internet. attached is the list mentioned above.in order for a compound to be considered optically active it can't have any axis or plan of symmetry. the molecule  shouldn't also  be superimposable on its mirror image.with such considerations, the compounds that are optically active are: -third compound on the first image is optically active; -second and third compound from the second image are optically actives.