Python中列表嵌套列表

在Python中
a=[ [1,2],[3,5],[4,6],[7,8] ]
那我想打印输出结果为1,3,4,7怎么做

b=[x[0] for x in a]
print(*b)

a=[ [1,2],[3,5],[4,6],[7,8] ]
for x in a:
    print(x[0])
    # print(x[0], end=",")

a=[ [1,2],[3,5],[4,6],[7,8] ]
r = map(lambda x:  x[0], a)
print(*r, sep = ',')