更改为默认数学字体

Question

之前的代码\begin{document}如下，本文档中数学字体不是默认字体，如何改成默认字体？

\documentclass[twocolumn,landscape,UTF8]{ctexart}
\usepackage{lastpage}
\usepackage{color}
\usepackage{wrapfig}
\usepackage{harpoon}
\usepackage{pgfplots}
\usepackage{tikz,tikz-3dplot}
\usetikzlibrary{shapes.geometric, arrows}
\usepackage{ulem}
\usepackage{titlesec}
\usepackage{graphicx}
\usepackage{colortbl}
\usepackage{listings}
\usepackage{makecell}
\usepackage{indentfirst}
\usepackage{fancyhdr} 
\usepackage{float}
\usepackage{setspace}
\usepackage{bm}
\usepackage{siunitx}
\usepackage{amsmath,amsfonts,amsmath,amssymb}
\usepackage{txfonts}
\usepackage{enumerate}
\usepackage{tikz,pgfplots}
\usepackage{tkz-euclide,pgfplots}
\usetikzlibrary{automata,positioning}
\usepackage[paperwidth=36.8cm,paperheight=26cm,top=2.5cm,bottom=2cm,right=1cm]{geometry}
\usepackage{background}
\usepackage[firstpage]{draftwatermark}
\SetWatermarkText{}
\SetWatermarkLightness{0.85}
\SetWatermarkScale{0.4}
\SetBgContents{}
\lstset{language=C,keywordstyle=\color{red},showstringspaces=false,rulesepcolor=\color{green}}
\oddsidemargin=0.5cm
\evensidemargin=0.5cm

\textwidth=30cm

\newsavebox{\zdx}

\newcommand{\putzdx}{\marginpar{
    \parbox{1cm}{
        \vspace{-1.6cm}
        \rotatebox[origin=c]{90}{
            \usebox{\zdx}
        }
    }
}}

\newcommand{\blank}{\uline{\textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}\ \textcolor{white}{a}}}

\newcommand{\me}{\mathrm{e}}
\newcommand{\mi}{\mathrm{i}}
\newcommand{\mj}{\mathrm{j}}
\newcommand{\dif}{\mathrm{d}}
\newcommand{\bs}{\boldsymbol}
\newcommand{\ds}{\displaystyle}

\newcommand{\fourch}[4]{\\\begin{tabular}{*{4}{@{}p{3.5cm}}}A.~#1 & B.~#2 & C.~#3 & D.~#4\end{tabular}}
\newcommand{\twoch}[4]{\\\begin{tabular}{*{2}{@{}p{7cm}}}A.~#1 & B.~#2\end{tabular}\\\begin{tabular}{*{2}{@{}p{7cm}}}C.~#3 &
        D.~#4\end{tabular}}  %两行
\newcommand{\onech}[4]{\\A.~#1 \\ B.~#2 \\ C.~#3 \\ D.~#4}  % 一行

\renewcommand{\headrulewidth}{0pt}
\pagestyle{fancy}