What is the first term a_1 and the common ratio r of the following geometric sequence if the second, third and fourth terms are: 2, 1, \dfrac{1}{2} ?

✓
a_1 = 4 and r = \dfrac{1}{2}

×
a_1 = 4 and r = \dfrac{1}{2}

×
a_1 = \dfrac{1}{2} and r = 4

×
a_1 = 4 and r = \dfrac{1}{2}