Set a default value in Dropdownlistfor

Question

In the form I have a:

@f.FormGroup().DropDownListFor(r => r.AtividadeId, Model.AtividadeList).WidthLg(6)  

listing all activities to perform a filter per activity.

I want to put an option: "List All" to list all activities without the filter. How I put this option and still leave as default?

On my model I put:

[Display(Name = "Atividade")]
[Required(ErrorMessage = "Campo obrigatório")]
public String AtividadeId { get; set; }
public SelectList AtividadeList { get; set; }

public IList<RelAtividadeVM> Atividade { get; set; }