Why is activity of solvated components equal to 1?

[crick]

Homework Statement

Consider the following reaction

$$\mathrm{Ni^{2+}_{(aq)} + H_2S_{(g)} \to Ni_{(s)} + S_{(s)} + 2H^{+}_{(aq)}}$$

What are the activities of the terms in the reaction?

Homework Equations

The activity is defined as $a = c/c^\circ$ in $\mathrm{ mol \, dm^3}$

3. The Attempt at a Solution

I do not understand why textbook gives $a_\mathrm{Ni^{2+} (aq)} =1$ and $a_\mathrm{2H^{+} (aq)} =1$: how can I say that those activities are $1$? Is there a rule to understand activites of solvated compontents (aq)?

 

[Borek]

It is not always possible, but you can rather easily prepare a solution containing Ni²⁺ and H⁺ in such concentrations that their activities equal 1. Makes sense as it is a standard state used as a reference point.

Something is wrong here, but it is hard to judge without a context. I would expect NiS(s) as a product, not mixture of elements.