According to the passage, which of the following statements about temperature gradients is true
A． These temperature gradients tend in most cases to be impossible to describe mathematically.
B． The lower the contrast between the extremes of the gradient, the lower the resulting velocity of the jet stream.
C． Jet streams are responsible for most of the north-south transfer of heat between the equator and the poles.
D． The temperature gradient tends, on a non-negligible level, to augment the effects of the earth's rotation.
E． The gradient typically refers to differences in jet stream velocity at various longitudinal positions around the globe.