参考答案

正确答案：D
解析：词语运用scratch one's head表：认真地思索一个难题；例：This crisis has politicians scratching their heads and wondering what to do．(这场危机令政治家们苦思不得其解。)；本句意为：这一现象仍令他们苦思不得其解。因此D为答案。