A man at a point P on the shore of a circular lake of radius 1 mile wants to reach the point Q on the shore diametrically opposite P (Fig. 16-5). He can row 1.5 miles per hour and walk 3 miles per hour. At what angle $\theta $ ($0<\theta < \pi /2$) to the diameter PQ should he row in order to minimize the time required to reach Q? <image 1>
['$\\theta = \\pi /2$', '$\\theta = \\pi /3$', '$\\theta = \\pi $', '$\\theta = 2\\pi $']