What distinguishes an R-controlled vowel?

An R-controlled vowel is characterized by the presence of the letter "R" immediately following a vowel, which alters the way that vowel is pronounced. In this case, the vowel sound is neither short nor long but is modified by the influence of the "R." For example, in words like "car" or "bird," the "R" affects the vowel sound, creating a unique sound that is distinct from the typical short or long pronunciations of the vowel alone.

The influence of "R" on the vowel is what primarily defines the R-controlled vowel, making the statement that the vowel comes before an "R" the correct choice in this context. Other options do not correctly capture this unique phonetic quality, as the presence of a diphthong, a silent vowel, or a doubled vowel does not fit the specific definition of R-controlled vowels.