单选题 Given the persistent and intransigent nature of the American race system, which proved quite impervious to black attacks, Du Bois in his speeches and writings moved from one proposed solution to another and the salience of various parts of his philosophy changed as his perceptions of the needs and strategies of black American shifted over time.
A. vigilant B. inadvertent C. impenetrable D. incongruous

【正确答案】 C

【答案解析】[解析] impervious意为“无动于衷的，不受影响的”。C选项：impenetrable意为“不能穿过的，不可理解的”，与画线的单词意义相符，如：The language of this document would be impenetrable to anyone except a specialist．(除了专家，其他任何人都看不懂这份文件。)根据句意：鉴于美国种族制度的持续性和顽固性，该制度证明是完全不受黑人抨击的影响的……。故选C。其他三项：vigilant警惕的；警觉的；inadvertent粗心大意的；incongruous不协调的，不和谐的。