Which of the following is an arithmetic series?

• a$1 + 2 + 4 + 8 + \cdots$
• b$1 + 3 + 5 + 7 + \cdots$
• c$2 + 6 + 18 + 54 + \cdots$
• d$1 + \dfrac{1}{2} + \dfrac{1}{4} + \cdots$