The SAT Question of the Day below turns on Harbrace Rule 20b: Choose expressions that are idiomatic.
An idiom is an expression whose meaning is peculiar to the language or differs from the individual meanings of its elements. Be careful to use idiomatic English, not unidiomatic approximations.
"She talked down to him" is iodiomatic.
"She talked under to him" is not.
Occasionally [as below] the idiomatic use of prepositions may prove difficult. [Check out the list of a dozen common "problem" usages in this section.]
Most ships move through the Suez Canal
. . . . . . . . . . . A
power, but large ships
must be assisted by a tugboat.
. . . D