to the left at the crossroads and you will see the railway station on your right.A. Turn

问题补充：

to the left at the crossroads, and you will see the railway station on your right.

A. Turning

B. If you turn

C. Turn

D. Having turned

答案：

A. Turning

B. If you turn

(答案→)C. Turn

D. Having turned

解析：句意，在十字路口向左拐，你会看到火车站在你的右边。and 连接了两个并列句，and之前是一个祈使句，所以用动词原形，故选C。