试题题干
Fill in each of the following blanks with a word or an expression.
Some transformational rules are obligatory and many are optional. The 
T-rules have to be applied if we want to obtain well-formed sentences.
参考答案
试题解析
一些转换规则是强制性的，但是大多数是选择性的，如果我们要获得结构完好的句子，就必须使用强制性的转换规则。