Which one of the following is not a microelement for plants?

Plants require many organic and inorganic nutrients to complete their life-cycle. It has been established that some elements are required by plants in relatively large amount and are called major elements or macronutrients, e.g., $\mathrm{C}, \mathrm{H}, \mathrm{O}$, $\mathrm{N}, \mathrm{S}, \mathrm{P}, \mathrm{K}, \mathrm{Ca}$ and $\mathrm{Mg}$. Microelements or minor elements or trace elements are required by plants in very small amount, less than 1 ppm. E.g., Fe, Cu, B, Zn, Mn, Mo, Cl. Thus $\mathrm{Cr}$ is not a microelement.