避免句子的头重脚轻指的是... 谢谢好心人指点！！！

句子要尽量避免头重脚轻，“头”指的是句子的主语，还是指一个句子的开头？

比如：

A.      With charcoal’s being replaced by coal as the fuel used in the smelting of iron ore and several improvements in blowing machinery, the average output of ironwork tripled over the eighteenth century.

这个句子算头重脚轻吗？

谢谢！