It is these poisonous products can cause the symptom of the flu, such as the headache and aching muscles.
A. who
B. that
C. how
D. what
网友回答
A. who
(答案→)B. that
C. how
D. what
解析：考查强调句：强调句的结构是it is/was+强调部分+that....，强调的是：these poisonous products，句意：是这些有毒的产品导致了流感的症状,比如头疼和肌肉酸痛。选B。