The drug used to treat this disease causes bad side effects, some of which deadly.
A. is
B. are
C. was
D. were
网友回答
A. is
(答案→)B. are
C. was
D. were
解析：B 考察主谓一致。Some of which引导非限制性定语从句，在从句中做主语，修饰先行词bad side effects，故从句中使用复数形式，由此排除AC，由主句的时态和语境可知表示客观性，故使用一般现在时。故B正确。