In 1858,a French engineer arrived in England with a plan for a twenty-one-mile tunnel under the English Channel. The tunnel would be well-ventilated if tall chimneys were built above sea level.后面一句是虚拟吗?与过去事实相反,应该是用完成时.为什么这里是用过去时.
这个tunnel将会有良好的通风，如果高高的chimney建在海面上