C++17中,构造路径对象正确的代码是 A. namespace fs=std:filesystem; fs::path b{"C:/Users"}; b = b / "cyd"; B. std::filesystem::path r{"C:\\Windows"}; r.append("System32"); C. std::filesystem::path r{"C:\Windows"}; r += "System32"; D. std::filesystem::path p{"/home"}; r += "cyd"; E. using fs=std:filesystem; fs::path p {"/home"}; p /= "cyd";