The sequence is $AGC \, ACA \, UUU \, AUG \, CCG \, AGC$. Which of the following options will not change the reading frame?

• A
  Addition of a nucleotide at the $9^{th}$ and $10^{th}$ positions
• B
  Addition of a nucleotide at the $3^{rd}$ position
• C
  Deletion of the $6^{th}$ and $7^{th}$ nucleotides
• D
  Deletion of the $10^{th}, 11^{th}$,and $12^{th}$ nucleotides