Making selection-screen block invisible / visible at runtime in SAP ABAP

REPORT ztestr_screen_change1.

PARAMETERS : p_matnr TYPE matnr.

PARAMETERS: ck_fl AS CHECKBOX USER-COMMAND chk.

SELECTION-SCREEN : BEGIN OF BLOCK b03 WITH FRAME TITLE text-019.

PARAMETERS : rd_up_a RADIOBUTTON GROUP r2 DEFAULT 'X' MODIF ID abc.
PARAMETERS : p_app_up TYPE dxfields-longpath MODIF ID abc.
PARAMETERS : rd_up_p RADIOBUTTON GROUP r2 MODIF ID abc.
PARAMETERS : p_pre_up TYPE dxfields-longpath MODIF ID abc.

SELECTION-SCREEN : END OF BLOCK b03.

SELECTION-SCREEN : BEGIN OF BLOCK b02 WITH FRAME TITLE text-018.

PARAMETERS : rd_dw_a RADIOBUTTON GROUP r1 DEFAULT 'X' MODIF ID abc.
PARAMETERS : p_app_dl TYPE dxfields-longpath MODIF ID abc.
PARAMETERS : rd_dw_p RADIOBUTTON GROUP r1 MODIF ID abc.
PARAMETERS : p_pre_dl TYPE dxfields-longpath MODIF ID abc.

SELECTION-SCREEN : END OF BLOCK b02.

AT SELECTION-SCREEN OUTPUT.

LOOP AT SCREEN.

IF ck_fl IS INITIAL.

IF screen-group1 = 'ABC'.
screen-active = 0.
ENDIF.

ENDIF.
MODIFY SCREEN.

ENDLOOP.