The centre distance between two meshing involute gears is equal to

• A. <img src="/_files/images/mechanical-engineering/machine-design/600-526-1.png">
• B. <img src="/_files/images/mechanical-engineering/machine-design/600-526-2.png">
• C. <img src="/_files/images/mechanical-engineering/machine-design/600-526-3.png">
• D. <img src="/_files/images/mechanical-engineering/machine-design/600-526-4.png">