According to SSTS No. 2, a tax return should be signed by a member only after reasonable

Question:

According to SSTS No. 2, a tax return should be signed by a member only after reasonable effort has been made to answer all questions on the return that apply to the taxpayer. What are some of the reasonable grounds under which a member may sign a return as the preparer even though some of the pertinent questions remain unanswered?