肾实质内肿块，表现为强回声，但后方无声影，是___________________较为特征性的表现。
参考答案：肾血管平滑肌脂肪瘤
—Do you like cats?
—Of course.They mean ________ a kind of pet to their masters.[ ]
A. less than
B. more than
C. no more than
D. not more than
双链DNA中没有配对的碱基被称为（）.
A.单排碱基
B.断链
C.粘性末端
D.缺链末端
