The bone will heal quickly after fracture, and the red bone marrow in cancellous bone plays a major role in the process of bone healing.

Analysis of test questions: The structure of long bone is as follows: ① cancellous bone, ② dense bone, ③ bone marrow and ④ periosteum.

A, the red bone marrow in cancellous bone has lifelong hematopoietic function and has nothing to do with bone healing; B, there are no osteoblasts in the compact bone, which are located in the periosteum; C, the red bone marrow in the bone marrow cavity has hematopoietic function and has nothing to do with bone healing, so A, B and C do not meet the question.

D, osteoblasts in periosteum have regenerative function, which has an influence on bone healing and bone length after fracture, so it conforms to the meaning of the question.

Comments: This is a basic problem, which is generally difficult. The key to solve this problem is to master the basic structure and function of bones skillfully, and remember them by combining the schematic diagram of bone structure.