Question

Both capital and natural resources can be used to produce other goods and services, but only capital resources:

Answer 1

Capital resources are goods made and used to produce other goods and services. They are the man-made physical resources. Some examples of capital resources are buildings, machinery, tools and equipment.
Natural resources are those resources created by nature. Some examples of natural resources include coal, aluminum, and gold.Foods are also considered natural resources.
Both capital and natural resources can be used to produce other goods and services, but only capital resources must be already produced. 
Capital may thus include physical goods and intellectual discoveries. Any resource is capital if it satisfies two criteria:
The resource must have been produced.
The resource can be used to produce other goods and services.

Answer 2

Only capital resources can help to transform with the use of skilled labour, the natural resources so they can be in a usable form to produce other goods and services. For example an open pit  copper mine, will use the skilful operators to load and haul and concentrate the mineral from which copper is extracted and then the smelter workers will refine the copper metal and then that copper can be used for wiring houses for example to provide electricity for both residential and commercial uses.