Python+selenium自动化测试：报错：TypeError: 'WebElement' object is not iterable

代码：

# -*- coding: utf-8 -*-
from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.support.ui import Select
from selenium.common.exceptions import NoSuchElementException
from selenium.common.exceptions import NoAlertPresentException
import unittest, time, re

class KeywordsAnalyse(unittest.TestCase):

driver=webdriver.Chrome("E:chromedriver_win32chromedriver.exe")

driver.get("https://。。。。。。/oldriver/login")


driver.find_element_by_id("username").clear()

driver.find_element_by_id("username").send_keys("")

driver.find_element_by_id("password").clear()

driver.find_element_by_id("password").send_keys("000000")

driver.find_element_by_xpath(".//*[@id='loginForm']/input").click()

driver.find_element_by_xpath(".//*[@id='side-menu']/li[4]/a").click()

driver.find_element_by_xpath(".//*[@id='keywords']").send_keys("fidget spinner")


select=Select(driver.find_element_by_id("category"))
all_options=driver.find_element_by_id("category")

for option in all_options:

print("")

driver.find_element_by_id("btnSubmit").click()


报道错误：TypeError: 'WebElement' object is not iterable

修改：

all_options=driver.find_element_by_id("category")改为

all_options=driver.find_elements_by_id("category")