Please! David has several chains of length 5 and of length 7. By joining chains one after the other, David can create different lengths. Which of these lengths is impossible to make? A)10 B)12 C)13 D)14 E)15

Answer:
13
Step-by-step explanation:
A)5+5=10
B)5+7=12
C) impossible
D)7+7=14
E)5+5+5=15