The medicine he had taken ____ the blooda.has a bad effect on b.had a bad effect on为什么选B

The medicine he had taken ____ the blooda.has a bad effect on b.had a bad effect on为什么选B
The medicine he had taken ____ the blood
a.has a bad effect on b.had a bad effect on
为什么选B

The medicine he had taken ____ the blooda.has a bad effect on b.had a bad effect on为什么选B
定语从句的时态 had taken 是过去完成时
根据意思 主语的时态应该是过去时