The following are important steps in the process of endochondral ossification.
1. enlarged chondrocytes die.
2. osteoblasts replace calcified cartilage with spongy bone.
3. chondrocytes enlarge and the surrounding matrix begins to calcify.
4. blood vessels invade the perichondrium.
5. perichondrial cells become osteoblasts and produce a thin shell of bone.
the correct order for these events is

1. Chondrocytes enlarge and the surrounding matrix begins to calcify

2. Enlarged chondrocytes die

3. Blood vessels invade the perichondrium

4. Perichondrial cells become osteoblasts and produce a thin shell of bone

5. Osteoblasts replace calcified cartilage with spongy bone

Explanation:

Endochondral ossification starts with the formation of the cartilage model. Chondroblasts become buried in the extracellular matrix of cartilage and are called chondrocytes. The growth of the cartilage model is accompanied by an increase in the size of chondrocytes.

The extracellular matrix begins to calcify. Other chondrocytes present within the calcifying cartilage become dead due to nutrient deprivation. The dead chondrocytes leave the space called lacunae. A nutrient artery penetrates the perichondrium and calcifying the cartilage model. It enters through nutrient foremen.

The perichondrium begins to differentiate into osteoblasts which in turn deposit the bone extracellular matrix over the remaining calcified cartilage to form a spongy bone.

correct answer is a. and d.