Consider the following statements: Which combination of these statements is correct?

1. If a set has $2n$ elements, the number of its subsets is $4^n$
2. $\mathbb{Q} = \{x : x = p/q,\ p \in \mathbb{Z},\ q \in \mathbb{Z}^+\}$
3. For $a, b \in \mathbb{R}$, $(a, b) = \{x \in \mathbb{R} : a < x < b\}$

• ai and ii
• bi and iii
• cii and iii
• di, ii and iii