单选题 Choose the correct statement about the "wetting" phenomenon.
   A．A fiber can be properly wetted by a matrix only when the surface energy of matrix is greater than that of the fiber
   B．A fiber can be properly wetted by a matrix only when the surface energy of matrix is lower that that of the fiber
   C．A fiber can be properly wetted by a matrix only when the surface energy of matrix is equal to that of the fiber
   D．It is nothing to do with surface energies of matrix and fiber