(a)

Observe the scatter plots in a graph,

The plotted dots are look like a periodic function, it is either a sine or cosine function.

From the graph, when  the value of  is maximum.

So the function of the graph represents a cosine function.

(b)

Observe the scatter plots in a graph,

The plotted dots are appeared as decreasing in a linear way.

So the function of the graph represents a linear function.

(a) Cosine function.

(b) Linear function.