According to the passage, which of the following statements is TRUE
A．Linguists have found that sign languages and spoken languages differ from each other in many features.
B．Like spoken languages, which use units of form to produce words, sign languages use units of sounds.
C．Separately, b, e, and t have a meaning and together they form the word bet.
D．Spoken languages contain units of form that by themselves hold no meaning, but when combined create a word.