Answer: b. The instrument is correlated with x1.
d. The instrument does not directly influence y, except through x1.
Explanation:
Based on the information given in the question, the necessary characteristics of a suitable instrument include:
• The instrument is correlated with x1.
• The instrument does not directly influence y, except through x1.
Some of the criteria for an instrument variable are the fact that it should have a causal effect on independent variable and also the dependent variable isn't directly affected except through the independent variable which is x1 in this scenario.
Therefore, the correct option are B and D.
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