Final answer:
By 2 months, infants can hold their head steady and follow objects. By 4 months, they can roll, sit up, and laugh.
Step-by-step explanation:
By 2 months, an infant can hold their head steady when held upright, follow an object in 180 degrees of motion, and coo and babble using vowel sounds. This is a normal part of their development at this age. By 4 months, they can roll from front to side, sit up with support, and laugh. These milestones are important markers of their motor and communication skills.