21. For Kozak sequence, which positions and the corresponding bases are proved to be important for the translation efficiency if the underline of AUG as the +1? (A) -4 (C/U) and +4 (U) (B) -3 (G/A) and +4 (G) (C) -10 (UAUA) and -25 (G) (D) -10 (UAUA) and -35 (U)