GWD 一道改错题~~求教it做代词的位置~~help~~~

weijiejing

Seldom more that 40 feet wide and 12 feet deep, but it ran 363 miles across the rugged wilderness of upstate New York, the Erie Canal connected the Hudson River at Albany to the Great Lakes at Buffalo, providing the port of New York City with a direct water link to the heartland of the North American continent.
A.Seldom more than 40 feet wide and 12 feet deep, but it ran 363 miles across the rugged wilderness of upstate New York, the Erie Canal connected
B.Seldom more than 40 feet wide or 12 feet deep but running 363 miles across the rugged wilderness of upstate New York, the Erie Canal connected

这道题A错是因为it没有指代对象还是因为but用的不恰当呢? 如果but用错了, 是错在哪里呢? it 是不是只能放在逻辑主语之后,还是可以放在逻辑主语之前?