text: function tabuada() {
      let num = document.getElementById ('txtn')
      let tab = document.getElementById ('seltab')
      if (num.value.length == 0) {
          alert('Por favor, digite um numero!')
      } else  {
          let n = Number (num.value)
          let c = 1
          tab.innerHTML = ''
          while (c <= 10) {
              let item = document.createElement ('option')
              item.text =  `${n} x ${c} = ${n*c}`
              item.value = `tab ${c}`
              tab.appendChild(item)
              c++
          }
      }
  
      
  }