Are forests a renewable resource? are they ordinarily used in a renewable way? how can forests be used more sustainably?

Trees are renewable because they can be replenished in a reasonable time frame. It takes too long to 'produce' fossil fuels, so they're not renewable. Thing is, the way we're using trees in most parts of the world is not sustainable. Forests are NOT renewable when they're clearcut or severely reduced. Forests are an ecosystem. When it collapses, species go extinct and don't come back. While we can plant more trees, we can't restore the land to the way it was. Sustainable forestry practices result in a consistent ecosystem which keeps the forest/trees renewable.

c. earths continental crust is thicker than its lithosphere.

because, the lithosphere is the outermost layer of the earth/ 100 km thick and is defined by its machanical properties.the lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. the lithosphere is the coolest layer of the earth in terms of temperature. the term lithosphere should not be confused with the use of "geosphere" which is used to indicate all of earth systems, including the atmosphere, hydrosphere   and biosphere. the surface of the earth can be divided into the continental crust and the oceanic crust.   the continental crust is made mostly of granite while the oceanic crust is made of basalt. the average thickness of the continental crust is 25 miles and the average thickness of the oceanic crust is 5 miles.   three types of rock, igneous, sedimentary, and metamorphic, make up the surface of the earth. that's why 3 option is preferable.