输出前 20 个斐波那契数，每行 5 个，补全代码
f1, f2 = 0, 1
n = 20
for i in range(1, n+1):
if i == 1:
fn = f1
elif i == 2:
fn = f2
else:
fn = _______
f1 = f2
f2 = _______
print(fn, end="\t")
if _______:
print()
f1+f2, fn, i%5==0
f1, f2, i==5
fn, f1+fn, i/5==0
f2, f1, i%5==0