清除某一离屏canvas的左半部分的内容，然后绘制于当前canvas，下方需补全的代码是？// 此处为清除某一离屏canvas的左半部分的内容----- 补全代码处 -----context.drawImage(offcanvas, 0,0); A: offcanvas.clearRect(0, 0, canvas.width/2, canvas.height); B: offcanvas.getContext('2d').clearRect(0, 0, canvas.width/2, canvas.height); C: offcanvas.clearRect(0, 0, offcanvas.width/2, offcanvas.height); D: offcanvas.getContext('2d').clearRect(0, 0, offcanvas.width/2, offcanvas.height);