A coffee shop charges for Wi-Fi access based on the time used. The graph of the charge function $c$ for $0\le t\le 40$ is given below. Write the intervals of continuity of $c$.

$(0,10)\cup (10,20)\cup (20,30)\cup (30,40)$

$(0,20]\cup (20,40]$

$(0,10]\cup (10,20]\cup (20,40]$

$(0,10]\cup (10,20]\cup (20,30]\cup (30,40]$