图示等直杆AC，杆长为4a，材料的抗拉刚度为EA，受力如图，则杆B截面的水平位移为： 。<img src="http://p.ananas.chaoxing.com/star3/origin/9d027834bb41632541d4482c401bf4a4.png" /> A: <img data="%7B%22formula%22%3A%22%5C%5Cfrac%7B4Fa%7D%7BEA%7D%22%2C%22id%22%3A183317%7D" src="https://mooc1-1.chaoxing.com/ananas/latex/p/183317" /> B: 0 C: <img data="%7B%22formula%22%3A%22%5C%5Cfrac%7BFa%7D%7BEA%7D%22%2C%22id%22%3A173376%7D" src="https://mooc1-1.chaoxing.com/ananas/latex/p/173376" /> D: <img data="%7B%22formula%22%3A%22%5C%5Cfrac%7B3Fa%7D%7BEA%7D%22%2C%22id%22%3A398230%7D" src="https://mooc1-1.chaoxing.com/ananas/latex/p/398230" />