score:1

Accepted answer
 {/* First select */}
            <select ref="markRef" onChange={(e) => { this.getMark(e); }}> // pass event as e
              {this.state.marks.map(function (mark, index) {
                return (
                  <option key={index} value={mark.id}>{mark.name}</option>
                )
              }
              )}
            </select>

            {/* Second select */}
            <select ref="modelRef" onChange={(e) => { this.getModel(e); }}> //pass e
              {this.state.models.map(function (model, index) {
                return (
                  <option key={index} value={model.id}>{model.name}</option>
                )
              }
              )}
            </select>

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