Don’t start reading a book ________ you find that it’s one you can read with ease and understanding.

A．when

B．if

C．unless

D．though

硬膜外麻醉术中突然出现心率减慢，与哪项相对无关（）

A.麻醉平面过高

B.牵拉反应

C.全脊髓麻醉

D.低血压，缺氧

E.误吸