What is at risk when the grey literature is excluded from a meta-analysis?

What is at risk when the grey literature is excluded from a meta-analysis?

A. The risk is that studies with biases will be overrepresented.

B. The risk is that studies not published in English will be underrepresented.

C. The risk is that studies with nonsignificant results will be underrepresented.

D. The risk is that studies with weak effects will be overrepresented.