improve crossing

• 改造杂交

improve crossing的用法和样例：

例句

1. Hybrids are produced by the unnatural crossing of two species.
   杂交种是由两个物种非自然杂交产生的。
2. This kind of pear is the result of crossing two different breeds of pears.
   这种梨是另外两种梨的杂交品种。