问题
下面的二项式定理公式中，正确的是哪个？选项
[A]. $(a + b)^{n} =$ $C_{n}^{0} a^{n – 0} \cdot b^{0} +$ $C_{n}^{1} a^{n-1} \cdot b^{1} +$ $C_{n}^{2} a^{n-2} \cdot b^{2} +$ $C_{n}^{3} a^{n-3} \cdot b^{3} +$ $\cdots +$ $C_{n}^{k} a^{n-k} \cdot b^{k} +$ $\cdots +$ $C_{n}^{n} a^{n-n} \cdot b^{n} =$ $\sum_{k=0}^{n} C_{n}^{k} a^{n + k} \cdot b^{k}$[B]. $(a + b)^{n} =$ $C_{n}^{0} a^{n – 0} \cdot b^{0} +$ $C_{n}^{1} a^{n-1} \cdot b^{1} +$ $C_{n}^{2} a^{n-2} \cdot b^{2} +$ $C_{n}^{3} a^{n-3} \cdot b^{3} +$ $\cdots +$ $C_{n}^{k} a^{n-k} \cdot b^{k} +$ $\cdots +$ $C_{n}^{n-1} a^{n-n} \cdot b^{n-1} =$ $\sum_{k=0}^{n} C_{n}^{k} a^{n – k} \cdot b^{k}$
[C]. $(a + b)^{n} =$ $C_{n}^{1} a^{n-1} \cdot b^{1} +$ $C_{n}^{2} a^{n-2} \cdot b^{2} +$ $C_{n}^{3} a^{n-3} \cdot b^{3} +$ $\cdots +$ $C_{n}^{k} a^{n-k} \cdot b^{k} +$ $\cdots +$ $C_{n}^{n} a^{n-n} \cdot b^{n} =$ $\sum_{k=0}^{n} C_{n}^{k} a^{n – k} \cdot b^{k}$
[D]. $(a + b)^{n} =$ $C_{n}^{0} a^{n – 0} \cdot b^{0} +$ $C_{n}^{1} a^{n-1} \cdot b^{1} +$ $C_{n}^{2} a^{n-2} \cdot b^{2} +$ $C_{n}^{3} a^{n-3} \cdot b^{3} +$ $\cdots +$ $C_{n}^{k} a^{n-k} \cdot b^{k} +$ $\cdots +$ $C_{n}^{n} a^{n-n} \cdot b^{n} =$ $\sum_{k=0}^{n} C_{n}^{k} a^{n – k} \cdot b^{k}$
$(a + b)^{n} =$ $C_{n}^{0} a^{n – 0} \cdot b^{0} +$ $C_{n}^{1} a^{n-1} \cdot b^{1} +$ $C_{n}^{2} a^{n-2} \cdot b^{2} +$ $C_{n}^{3} a^{n-3} \cdot b^{3} +$ $\cdots +$ $C_{n}^{k} a^{n-k} \cdot b^{k} +$ $\cdots +$ $C_{n}^{n} a^{n-n} \cdot b^{n} =$ $\sum_{k=0}^{n} C_{n}^{k} a^{n – k} \cdot b^{k}$