D. every natural language is combinable system. = if it is not combinable system, it is not natural language.
Assumption的题目对选项取非,即 if it is combinable system, it is not natural language.
加上原文的条件 No A is combinable phonetic system.
推出 No A is not natural language.
与结论冲突: A is not natural language.
削弱了结论,所以D是正确答案。