They promised to develop a software package by the end of th

C

和difficult，difficulty搭配的状语从句的连词分别是however difficult, whatever difficulty, 根据they might have 可知，此处句型为sb have difficulty (in) doing something，因此排除A.B。句意为他们承诺在今年年底要开发一个软件包，无论可能会有什么样的困难。无论有什么样的困难，即whatever difficulty they have 或者no matter what difficulty they have,或者however difficult it might be, no matter how difficult it might be. 排除D项。故选择C项。
【考点定位】考察状语从句。