Which of the Following Statements Is TRUE of Restriction Enzymes

Which of the following statements is TRUE of restriction enzymes?

A) They are not naturally produced by bacteria, but are bioengineered by them.
B) They protect bacterial cells from invasion by foreign DNA.
C) There are less than 10 restriction enzymes known.
D) They cleave the DNA backbone at only one location.
E) They cleave DNA only at sites of adjacent thymine bases.