Which of the following sequences in double-stranded dna is most likely to be recognized as a cutting site for a restriction enzyme?
a. aagg
ttcc
b. ggcc
ccgg
c. acca
tggt
d.

b. GGCC

CCGG

Explanation:

Restriction enzyme cleaves DNA at the restriction site. Restriction site has 4-8 nucleotide base pairs which are recognized by the restriction enzyme. This site usually has a palindromic sequence which means that the sequence is read as same on both the strands in 5' to 3' direction. Palindromic sequence is used because it increases the chances that both the strands are cut. Enzyme is also able to recognize the sequence no matter from which side it approaches the DNA. The enzyme also works as homodimers sometimes so having the same sequence makes the dimers easier to recognize it. Here, the sequence is GGCC in 5' to 3' direction on both the strands making it a palindromic sequence. Hence it is most likely to be recognized as a cutting site for a restriction enzyme.  

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.