Table schema:
CREATE TABLE table_name_66 (height__cm_ INTEGER, birthdate VARCHAR, weight__kg_ VARCHAR, position VARCHAR)

Question: What is the average Height when the weight is less than 91 and the position is d, and a Birthdate of july 4, 1975?
SQL: SELECT AVG(height__cm_) FROM table_name_66 WHERE weight__kg_ < 91 AND position = "d" AND birthdate = "july 4, 1975"