His mother _______ at the age of 90.
A:passed by
B:passed away
C:passed down
D:came to herself
加入抗凝剂的血液离心后出现分层现象,正确的分层顺序(由上向下)是( )
⒈____ ⒉____ ⒊____ ⒋____ ⒌____ ⒍____