对于升序有序整数数组nums和目标值target，下列哪个Python实现能够正确找到第一个大于等于target的元素的下标（即左边界）？若数组中无目标值，则返回第一个大于target的元素下标，若所有元素都小于target则返回数组长度。

def left_bound(nums, target): left, right = 0, len(nums)-1 while left < right: mid = (left + right) // 2 if nums[mid] >= target: right = mid else: left = mid + 1 return left

def left_bound(nums, target): left, right = 0, len(nums)-1 while left <= right: mid = (left + right) // 2 if nums[mid] >= target: right = mid else: left = mid + 1 return right

def left_bound(nums, target): left, right = 0, len(nums)-1 while left < right: mid = (left + right) // 2 if nums[mid] < target: left = mid else: right = mid - 1 return left

def left_bound(nums, target): left, right = 0, len(nums)-1 while left <= right: mid = (left + right) // 2 if nums[mid] > target: right = mid - 1 else: left = mid + 1 return left