Regarding lanthanide contraction, some statements are given 

(A) It arises because of the poor shielding effect of 5f-electrons

(B) The atomic radii and ionic radii steadily decrease from Ce to Lu 

(C)  The crystal structures and other properties are similar 

 (D) The seperation of lanthanides from one another is difficult from their salt solutions. 

The correct statements are

Lanthanide contraction arises due to the poor shielding effect of 4f-electrons. Due to lanthanide contraction, atomic/ionic radius decreases when the atomic number increases from lanthanum(atomic number 57 to lutetium (atomic number 71). The crystal structure and other properties remain the same. It is difficult to separate lanthanide from one another from their salt solution due to similar properties.