The strongly acidic nature of DNA poses a problem. Why do Watson and Crick think that the DNA "dyad", formed by hybridization of two negatively charged molecules, is stable?

Negatively charged phosphate groups of two strands are aligned distantly from each other in a DNA double helix.

Explanation:

DNA double helix is formed when the complementary bases of two single DNA strands pair with each other. The formation of double-stranded structures places the negatively charged phosphate groups of two single DNA strands away from each other. This is because complementary base pairs are present between the sugar-phosphate backbones of two DNA strands of a double helix. The double-helical structure also concentrates the nitrogenous bases away from the surrounding watery medium. Altogether, these factors stabilize DNA dyad.

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.