2012年元月的某一天，我市的最低气温为－3℃，最高气温为4℃，那么这一天我市的日温差是

A．3℃

B．4℃

C．－7℃

D．7℃

设有如下程序：
Prirate Sub Form_Click()
Dim a As Integer,b AS Integer
a=20:b=50
p1 a,b
p2 a,b
p3 a,b
Print"a="；a,"b=";b
End Sub
Sub p1(x As Integeh By Val y As Integer)
x=x+10
y=y+20
End Sub
Sub p2(ByVal x As Integer,y As Integer)
x=x+10
　y=y+20
End Sub
Sub p3 (ByVal x As Integer,ByVal y As Integer)
　x=x+10
　y=y+20
End Sub
该程序运行后，单击窗体，则在窗体上显示的内容是：a=______和b=______。